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low RPM in 72 318

L

loid58

Contributing Member
After about 4 yrs of not having a working tach, I put in a petronix ignitor ignition. Hooked up the tach and now it works, 500 at idle. Question is I can only get about 1600 rpm at full throttle while running. I have always cruised at this speed, but just thought she's slow and thats all the boat had. Probably going about 18mph. Now I'm thinking i should get at least 3000 rpm. Did the Fast Jeff distrib. thing, everything in there seems fine no rust and it snaps right back. motor runs great, no misses or knocking, or overheating, rpm go to over 3000 in neuteral, timed it last year, right on. Has the original carter AFB, get that carter bog when accelerating but only for a second. Dont know when it was last rebuilt, (I'm afraid to take that carb off). Could it need a rebuild or would the 1409 carb get me up there in RPMs, or is the whole distrib shot? Would apreaciate any help
 
You need to re-time it, and do the wire pull test as well.

To wit: Idle at 1,000-1,500 warmed up. Using insulated pliers and heavy gloves, pull then replace one wire at a time from the distributor cap and listen to how the motor slows down. If it doesn't slow down leave that wire out (then find out why it's not working). Revealing test!

Jeff
 
Jeff tried the wire pull test, and everything is fine all cylinders slowed when pulled , getting spark, did the advance weight thing all seems fine. ran the boat throttle by hand and noticed the secondaries only opened about an eighth of an inch at WOT. couldn get them to open any more by hand, stuck a screwdriver down there to try to pry them open, but it seems like they,re stuck, thus starving for fuel wide open. Is this normal or are they supposed to open all the way? Seems to be the original carter on there.
 
Next.... calibrate your tachometer for accuracy. If inaccurate, your numbers are somewhat meaningless.

You mentioned that you timed the engine. Can you describe your method? IOW, are you setting BASE and calling it good, or are you taking it further and seeing where your TA is? (TA = Total Advance @ the limit RPM)
TA is of much more importance over that of BASE.
See your OEM specs.
Strobe the marked off balancer, and jot down the ignition advance at an array of RPM.
1k, 1.5k, 2k, 2.5k 3k and 3.5k rpm.
Plot this out on paper, and compare to your OEM specs.

NOTE: many OEM ignition curves will not include BASE advance, so when you take your real numbers, these will reflect BASE as being included.
You may need to do the math when comparing.


If you can't reach your WOT RPM spec, you'll risk over-burdening the engine at any RPM.

.
 
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That carb has a secondary lockout when the choke is on...check the linkage to be sure the lockout is not the problem.
 
The choke has to be fully open for the secondaries to kick in. Note that the actual butterflies ar at the bottom of the carb (assuming it's a Carter AFB). That 'thing' at the top is the flop over valve that acts like an accelerator pump.

Jeff
 
If you never got over 1600 rpms are you sure you have it propped right? A prop with too high of pitch will not allow it to obtain higher rpms. Just a thought.
 
capnlazy said:
If you never got over 1600 rpms are you sure you have it propped right? A prop with too high of pitch will not allow it to obtain higher rpms. Just a thought.
Click to expand...
This perhaps a "Cart ahead of the Horse" scenario.
But I too agree with the above.

However......
1600 rpm at full throttle while running................... Probably going about 18mph.
Click to expand...
1,600 RPM = 18 MPH????? Does this sound right to you guys?
This may also be a recipe for Detonation if not careful, IMO.

Here's the kicker:
In order to know if this boat is propped correctly, and/or geared correctly, a WOT RPM test must be performed.
In order to do this, the OEM WOT RPM specs must be known.
In order to perform a WOT RPM test according to these specs....., the engine must be in full tune, and in good health.
(see post # 5 again re; ignition timing)
The Tachometer must also be accurate, or the numbers are somewhat meaningless.
The hull must also be clean and free from any hull growth.
Prop must be in full tune as well.

Point being..... these ducks must all be in a row prior, or you will be spinning your wheels in the sand!

.
 
Timed the motor the usual way last year, now cant find timing light, think I lent it out. but marked dist where it is and moved dist each way till motor changed pitch then put it back to where it was. Get bogging when the throttle is quickly flipped, but smooth if steadily accelerated. get the Carter flat spot near full throttle then it goes away. The choke is wide open when running so the secondary lockout should not be a problem. as for prop? I have no idea the boat was given to me a few years ago and I had to put the motor and trans in. I'm sure its in need of tuning or replacement. Its a 25' luhrs 1972, glass hull w/ wood cabin, old heavy boat. What prop would be correct? estimating about 15-20 mph at WOT like ricardo said, just thinking thats all its got and gotta live with it, which is ok by me. Thanks for the replies, any more thoughts would be welcome. P.S do the secondaries open all the way or just the slight bit as I previously mentioned? They open maybe 1/8th on an inch.
 
loid58 said:
Timed the motor the usual way last year................. I'm sure its in need of tuning
Click to expand...
Man, I'm going to sound like a grumpy old guy here :mad: :).....:)

An Engine that is NOT receiving the correct spark lead (i.e., Ignition Advance @ a given RPM), is NOT going to perform..... period!
BASE advance is BASE advance all day long.
We fire on BASE..... we idle on BASE.
The curve and TA and TA RPM must be our concern if we want any performance at all!

Find your timing light..... mark off the Harmonic Balancer...... strobe this as you increase RPM.... jot down the numbers, and compare to your OEM Ignition Advance curve.


Common issues related to an old mechanical advancing ignition system:
  • No advance past BASE.
  • Advance, but no where near enough.
  • Irratic advance.
  • Advance coming on too late.
  • Advance coming on too early and too strong..... (this is the dangerous scenario)
Until you check to see where your ignition advance is, you're spinning your wheels.
 
Hold the phone, there is no way you are going that fast at that RPM so lets assume the tach is wrong. Do you have a handheld dwell/tach or a timing light with a rpm readout you can use to verify the actual rpm before you start tearing things apart? The tach could also be set for a 4 or 6 cyl engine.

Dan
 
I probably am not going quite that fast, you're right, just a very rough guesstimate. as for the OEM specs, i have no idea what those would be. Does anyone know? or know where to find them? Tach seems to be accurate at idle it reads 500 and engine sounds to be idling there, could be inaccurate, it is 39 yrs old after all. I still suspect somethings up with the secondaries, they only open slightly. Seem to remember from my hot rod days that secondaries opened up fully. On the carters do they fully open? checked linkage and its good
 
"as for prop? I have no idea the boat was given to me a few years ago and I had to put the motor and trans in. I'm sure its in need of tuning or replacement. Its a 25' luhrs 1972, glass hull w/ wood cabin, old heavy boat. What prop would be correct?"

I don't know what size prop would be correct. Do you know what size you have? I have a 24' Chaparral that weighs about 6000# and I think I have about a 14 1/4 X 17 prop. If you put the motor and transmission in this boat and you never got to WOT, I would think this would be something that would be easy to check.
 
Well, the motor was the same one that was in there, a LM318 225 hp. The trans was the one the boat came with, a borg warner velvet drive 71 I think?, and a walter Vee drive that the case was rotted out. I had the trans rebuilt and found a replacement walter with the same 1.46-1 ratio as the original. Have no idea what prop is on there. The previous owner is a neighbor, I'll ask him if he knows. If I can't figure it out , I'll just finish out the season and bring the prop to a propeller shop thats in my area. Still would like to know if the secondaries on the carter carb are supposed to open all the way at WOT. Anybody know?
 
"Still would like to know if the secondaries on the carter carb are supposed to open all the way at WOT."

Seems to me every automotive AFB I've worked on opened almost completely. And what sense does it make to have a 4 bbl if the secondaries don't open? May just as well have a 2bbl. It's been a while since I've had a Carter apart but my guess is the carb was not adjusted correctly when rebuilt / serviced. There is tons of info for the AFB on the web......................

http://www.mymopar.com/downloads/carterafbtuning.pdf

http://www.thecarburetorshop.com/Troubleshooting.htm

http://www.laroke.com/larryk4674/2009/AFB_Rebuild_Guide.pdf
 
Heres a diagram for the AFB carburetor;
http://www.laroke.com/larryk4674/2009/AFB_Rebuild_Guide.pdf
Part number 31 on page five is called the secondary auxilliary valve assembly...it is located above the mechanically operated secondaries and is designed to stay closed until the engine is pulling enough rpm and air volume to handle their opening. As Jeff pointed out, without this setup, you would need an extra accelerator pump for the secondaries(double pumper). If the engine is not running and you open the secondaries, you will not be able to visually see through to the intake manifold...your view will be blocked by the auxilliary valve. If you are at 3000 rpm and under light load, the air valve will open when the traditional secondaries are opened and you will hear the unmistakeable roar as the auxilliary valve opens.
 
loid58 said:
............ Still would like to know if the secondaries on the carter carb are supposed to open all the way at WOT. Anybody know?
Click to expand...
I don't mean for this to come off as a smart a$$ response, but of course they are to be fully open........ WOT = Wide Open Throttle!

The correct load determined by many factors, including over-all gear ratio and correct prop pitch, etc., control RPM..... hense the term; WOT RPM test.

.
 
Thanks guys, somethings up with the carb then, cause those secondaries only open about an 1/8th of an inch or less. Its like they,re stuck, even stuck a screwdriver down there, they wont budge.
The secondary auxilliary valve on top is freely operating, but those butterflies under them are stuck closed. Will mess with them this weekend and see if i can find the problem. Maybe if they work properly I can get better rpm and mph, and less fuel economy, ha ha.
 
Does the boat feel like it's going faster since you put on the new distributor..or does it feel the same as before?

18mph at 1600 rpm...no way

I installed a Pertronix ignitor on my '85 318s and I was getting 800 more rpms before I installed them.*

Still chasing my problem also.* Problem is, I've installed too many new things (distributor, 45k coil,*Edelbrock*1409*carb, spark wire(cheap*Chinese crap))......*so I can't pin point the problem child yet.*

Timed base at 7* with advance limited at 20*= Total advance of 27* Not sure on oem curve but*all advance should be in by 2500 rpms.

Pertronix chart is in here http://www.pertronix.com/support/manuals/pdf/billet.pdf

Props.....If I can reach my 3300 rpm I had before.... my props are fine.


Still searching...and I will update if I find out anything.:) I think the new distributor, cheap plug wires, or secondaries are not opening all the way as you said will be my culprit.
 
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tundrarules said:
Problem is, I've installed too many new things (distributor, 45k coil,*Edelbrock*1409*carb, spark wire(cheap*Chinese crap))......so I can't pin point the problem child yet.

Timed base at 7* with advance limited at 20*= Total advance of 27* Not sure on oem curve but*all advance should be in by 2500 rpms.

Pertronix chart is in here http://www.pertronix.com/support/manuals/pdf/billet.pdf
Click to expand...
This is very common in that when we throw a bunch of parts at an issue, we will eventually cure it.
It becomes how much do we spend, and how large is the carry-out box?
Problem is, we will not necessarily know which part did the trick.
The P of E (process of elimination) is always our best bet.
However, it must be done systematically and methodically one item, and ONLY one item, at a time. :)

As for your Ignition specs, be sure to use OEM Specs, and not specs from other sources unless you are confident that these are accurate.
Examples would be the S__oc and Cl__mer manuals.
Many of us have read and seen the occasional errors in these manuals before. Perhaps not often, but occasionally!

Total advance of 27* Full In by 2.5k rpm would not be recommended for a SBC Marine Engine (the 27* is not the issue.... the 2,500 rpm is!)
However, if this is suggested for the Chrysler 318/360 Marine Engine......, I'd want to verify this.
Best way ??? .... OEM specs!!!! :cool:

Edit:
BTW, the Pertronix pdf file that you linked us to, shows an automotive ignition distributor.
If this indicates that the information is for Automotive, I'd caution you.
We DO NOT want to use any Ignition Advance information that pertains to Automotive!

.
 
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loid........sometimes those secondary valves will hit on the intake or the gasket you put in between if its not placed on there perfectly.....then check for movement......then torqued ....then check for movement again........and that linkage has to be move for the secondaries to open (air valve he mentions above)
 
I am reading this thread hoping it will tell me how to fix my 3 years of frustration. I have gone as far as putting a long block in and still have issues with low rpms / low power. The port engine jumps to life and pushes 4000+ rpms but the starboard still only gets to 3200 to 3400 rpms. I believe it is running on 7 cylinders. It has a Mallory distributor, a stock coil, a 1409 carb that was just rebuilt.

So I am getting fuel, the engine (long block) is new, the prop and shaft are aligned and correct.

To keep the engine idling the timing was at 20 advance (used an adjustable advanced timing light) and I could not drop it down to 5 degrees. I used the front balance mark and timing plate on the block.

If the engine was only firing on 7 cylinders would advancing the timing keep it running?

I am going to put in new plugs and wires just to keep my sanity.
My next steps are to start measuring electrical values between engines. Where should I start?
- distributor (What do I look for?)
- ground system (Check block to ground terminal)
- ballast resister?
- ???

Jeff
1978 Penn Yan 26'
twin 318s
 
Jeff, it may be best to begin a fresh new thread, rather than hy-jacking an existing thread.
But.... since we're here..........

First and foremost when it comes to ignition timing, the harmonic balancer must be accurately marked off and in phase with the crankshaft angle relative to #1 cylinder.
The outer ring on the balancer is rubber dampened.
These can occasionally slip and change position regarding original indexing.
The actual markings are on the the outer ring....., so you can see how this would affect ignition timing if it were to have slipped.

Ideally, something like this is what you'll want for a Std LH rotation engine.
It will be just opposite for a REV RH rotation engine.
images
...
15807028_130x130.jpg


Your BASE advance of 20* is simply incorrect. Something is definitely wrong here.
Either the balancer outer ring has slipped and you are NOT actually seeing 20* advance......., or the camshaft is not indexed to the crankshaft correctly.
In your post you mention using the advancing type timing light.
Is there a chance that this timing light has a glitch?
Is there a chance that you may not be using it correctly?
I'm not a fan of using the Digitally Advancing type timing lights for our Marine purposes. This is just too dang important to risk user or equipment error, IMO.
I want to see this advance in Real Degrees and in Real Time.... and this is why the balancer must be marked off, and accurately.



So........ IMO, first on your list will be locating true TDC for #1 cylinder.
Second will be seeing if the camshaft is phased correctly.
Assuming that this will be verified and/or corrected, the next thing will be the progressive ignition advance.
Without a prescribed progressive ignition advance (i.e., curve, limit and full in TA rpm), you will not be able to pull the potential power from this engine.
Do NOT settle for BASE advance only!
Take it further and see where your TA is.... and I'd suggest doing this with a standard strobe light.
Correct ignition timing is the only way that you'll get the max from your engine.

I'd certainly get this out of your way first!
Once these are verified, you can move on to other potential issues.



Jeff, I think that it's still best to start a new thread on this.
Most of us will see it and offer our help to you.

.
 
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Well guys got the secondaries working, the linkage rod from the vacuum dashpot to the choke was out of wack and not letting the secondaries open, just bent the rod and now they work. But, still cant get good speed , don't know whats up, still only at around 1600 rpm. I'll just ride out the season, i can deal with it. thinking maybe a messed up prop, will look into it when hauling out for painting and cutlass bearing in the spring. Thanks for all the help
 
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