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PCM Chevrolet 305 Engine Dies above 1400 RPM

R

robin_schouten

New member
I recently own a Chris Craft C

I recently own a Chris Craft Commander 31 (form approximately 1974) with 2 PCM Chevrolet 305 Engines new installed in approximately 1992.

The left engine works fine but the right engine backfires and dies with more than appr.1400 RPM under load. And I cannot make this engine run OK

Since then I have had the Holley carb.completely rebuild. Put in new ignition points + rotor + condenser + cap + spark plugs + coil.
Set the points with a dwell meter on 28 degrees (works better for me than a feeler gauge) and timing 9 degrees BTDC (as in the manual) on flywheel with strobe scope .

The (cold) engine starts immediately runs perfectly smooth on idle but under load it holds back enormously
when I go over 1400 RPM throttle. (no more backfires after working on the ignition)
If I quickly reduce throttle (under 1000 RPM) the engine will return to normal if i keep the same or higher throttle the engine dies completely

I have checked the ignition (with the strobe scope lamp) and the plugs are still firing when the engine dies
I also mounted the carb. from the good working engine and the problem was still the same. (so rebuild carb. is good).
There is a clear tube on the fuel pump but there is no fuel in it so I think this is working ok.....

I still think that it is a fuel problem but I don't know what to do anymore.
Does anyone have any suggestions for me?
Thank you for your time and if you have any suggestions i will be very happy!.
 
Vent line and fitting clear an

Vent line and fitting clear and unobstructed?

Fuel shut off valves completely open?

anti-syphon valve fault?

Loose clamp on fuel line or cracked fuel line admitting air?
 
"Tomorrow i will check your su

"Tomorrow i will check your suggestions and also
bypass the fuel supply by connecting a "fuelbottle" straight to the fuel pump.
Wil check the vent line, there is no shut off valve and i will check the clamps and rubber lines. But what is an anti-syphon fault?
Thanks for your help so far..."
 
"Anti-syphon valve - a check v

"Anti-syphon valve - a check valve mechanism usually placed in the fuel line at the tank pickup point. Typically with an aluminum colored finish. Some would say it looks like a pipe (threaded) extension.

The ball in the check valves have been know to corrode and 'freeze up', restricting flow. Other failure modes have been documented. I usually pull them and remove the ball with a punch. Then reinstall with pipe sealant on the threads."
 
Bypassed the fuel system and t

Bypassed the fuel system and the problem was still there. So no problem with the flow to the fuel pump! Maybe the fuel pump itself but i have to make a T-split in the alu pipe that runs from the pump to the carb. to be able to measure the pressure. I think it has to be between 5 and 7 PSI????
I also can swap the (mechanical)fuel pump from the other engine but is that difficult or should i do or need something special to do that?
Or just unbolt it en thats it?
 
"4 psi is plenty, at the carb.

"4 psi is plenty, at the carb.

I'd hope the pressure check is easier than a pump swap (and faster, too). Those pump are over 15 yrs old and are probably due for a change. Some have run for 20 years but I don't recommend it.

You need a new gasket and some patience to swap the pumps. easiest to rotate the crank go minimize the pressure on the pump arm. makes slipping behind the pushrod easier. think there's a counter bore you can use a pinch bolt with but its been a few years since I've done a small block"
 
This problem you are having is

This problem you are having is the exact same problem I am having with a 92 Ski Brendella with a PCM 351 engine. Runs like a champ up to 1600 RPM and then labors as if not getting enough fuel. Holley 650 Carb. has been completely rebuilt. Ignition has been overhauled completely except for the plug wires. This is driving me totally nuts. Any help will be greatly appreciated.
 
I am going to change the plug

I am going to change the plug wires also.
I dont think that can give such a major problem but they are in bad shape so to rule this out also. Are these "special marine" type or can i just get universal chevy v8 8mm wires from the car shop???
 
"measure the fuel pressure yet

"measure the fuel pressure yet? Most of those holley's have fuel level inspection ports so you can check the fuel level within the carb - may wanna check it, too.

If you used an outboard tank for the alternate supply, and it has a primer ball in the hose, you may wanna take it back, too. You can use that primer ball as a surrogate fuel pump when the bog sets in.

The wires are same, material wise - what is usually different is the length due to the different routing needs on a marine engine"
 
"Cant make an easy connection

"Cant make an easy connection for measuring the fuel pressure for now.
(Cant find it on the Holley 4160 as well)

I have seen that the Nr. 1 plug wire is NOT at 6:30 position seen from the flywheel, but its on the other side of the cap.......
I dont know if that can cause a problem....???
(On the other (BB) engine its on 6:30)

Maybe its possible to set Piston 1 at TDC and look where the rotor is and mark that as Nr.1 Wire and work the firingorder from that point on ???? Or is this creating my problems with the engine??"
 
"Not sure what (BB) &#

"Not sure what (BB) (On the other (BB) engine its on 6:30)is getting at. You mentioned manual before so let's assume you have the manual....

The absolute position of the number 1 plug lead on the distributor isn't critical. That said, it will be easier on everybody, in the future, to use the 'standard' position - which should be in your manual (6:30). To find the right spot, you need to set #1 to TDC, but on the compression stroke. pull the plug or the rocker cover to make sure this happens.

Once that is done, you need to ascertain if the engine is RH or LH (standard or reverse rotation). that will determine what order the plug wires following number 1 go in. Again, check your manual or the engine itself, while cranking. All this should be in the manual. You can use that info to see what the current order is, too, to see if you have to redo it. (the reference is determined looking at the flywheel-CW = RH, CCW = LH. If you look from the front, it will be 'backwards'.)

Also, be very careful with the wires - too much hard flexing will break the inner cores and cause problems down the road. Also, make sure the wires seat tight on the cap and the plug. Finally, make sure the correct plug (cylinder number) is selected. Having another set of eyes helps, too, as does a set of wire marker labels/clips (if available)."
 
Will first change the wires wi

Will first change the wires with new ones and see what happens.
(when you put your hand on the wires on the cap and ground you get a shock and the resistance in ohms is on 2 wires 17kOhm and the other 6 almost zero so that cant be good)
Then i go further with the fuelpump (maybe a swap with the fuelpump of the other engine) is the best test)
 
"The shock is due to an "e

"The shock is due to an "electrical leak". To minimize it, use a good silicone dielectric grease in the boots on both ends of the wires. A thin layer is better than a thick layer.

Guessing the resistance is that of the spark plug lead - terminal to terminal - depends upon the type of lead and its length. A nominal value of 4K ohm per foot used to be the norm (another manual check may help here). I'd be mmore worried about them being different. If your ignition system wants resistor type spark plug cables, don't use anything else (ie solid wire types).

Those Holleys should have sight plugs, so you can see if the fuel level is maintained in the bowls. If you don't know where they are, get the exploded diagram from holley's www site. Checking those plugs is much easier than the pump swap."
 
Is there a way to find out whe

Is there a way to find out whether i need resistor spark plug wires or not?
(This is not mentioned in the manual)
Because it seems there are both kind used.
(The ignition is a plain old Mallory point ignition)
I know what you mean but unfortenately there are on this type of Holley no sight plugs on the side of the fuel bolws.
 
I'd be inclined to go with

I'd be inclined to go with the resistance wire...you could always check with Mallory in the morning.
 
Have new wires will put them o

Have new wires will put them on tomorrow.
Can it be there's something wrong with the advance system in the distributor?
Have checked it and it's complete and not stuck.
Also looked with the strobe and when i give more trottle the marker on the flywheel advances......
 
Lack of advance will keep you

Lack of advance will keep you from achieving elevated RPM levels - your description indicates it is functioning.
 
"Put on the new wires and no e

"Put on the new wires and no effect.
I have noticed that there is some space in the axe (where the rotor is on) thats in the distubutor! (Approx. 0,04 inch)
Maybe with more rpm its "wobbeling" more so that the points gap changes ?????
However on idle its runs smooth an the timing (checked with strobe) doesnt "dance" around....
Will check first whether the dwell changes with more RPM. I think that when the axe is worn out and moves to much the dwell changes also."
 
"Are you talking about the dis

"Are you talking about the distributor shaft and it having radial play (moves when you shake the rotor left -right or front-back)?

If loose, will it change the points' gap and the dwell angle and cause issues on the high voltage side, too."
 
Yes I couldnt find the right w

Yes I couldnt find the right words for it (i'm from the netherlands) but it has radial play on the rotor shaft.
I think i need a new distributor as well......
Maybe choose for an electric model.....????
Is it possible to fix this?
Maybe the distributors are swapped from the original engines....
 
Checked the fuelpressure and t

Checked the fuelpressure and that is NOT constant.
On idle its 5.5 PSI but above 1400RPM in gear (when the engine holds back or dies the pressure drops to almost zero. So either the pump is bad or maybe with the camshaft or rod that drives the pump (if that is possible)
Will swap the pump from the other engine to see what happens....
 
"So your problem is inadequate

"So your problem is inadequate fuel supply.

Could be the pump or the cam lobe. Could also be a clogged filter, before the pump OR the supply hose is collapsing OR a loose clamp or fitting. If that engine has it own tank, check the vent to make sure it is open. I'd check these BEFORE switching the pumps between engines.

Your English is pretty good."
 
I have bypassed the system wit

I have bypassed the system with a small external tank straight to the pump and the problem was still there.
Then placed a clear tube between the pump and the carb. and on idle pressure(5/6 PSI)and fuel are ok but in gear above 1200RPM the pump is giving air bubbles and not much fuel and the pressure drops to 0 PSI.
When going to idle the air bubbles disappear and pressure is going back up to 5/6 PSI
So therefore i think it cannot be a problem otherwise than a faulty pump.
Will search here for buying the same pump (carter) and mount that and see what happens.
 
Sounds pretty reasonable to me

Sounds pretty reasonable to me....

You may want to consider getting two; the other side is probably not too far away from doing the same thing.
 
"Dick,
I saw your posting

"Dick,
I saw your posting about your issues with your engine. I have a 351 Pleasurecraft in my supra, and it stalls on me after I go for about 10 seconds at anything greater than 2k rpm. It started out at 3K, but now it seems to be doing it at anything over 1500 RPM. It starts and idles fine....but it seems to be taking longer to restart between stalls, as if the fuel pump needs time to prime. I had an electrical issue that shorted everything out, so I just replaced the starter, alternator, solenoid, and I had the carburetor rebuilt. When does it all end? I was going to replace the fuel pump, but I wanted to get your input. It sounds like you know a lot and you did a lot of great troubleshooting. I never found out if the fuel pump replacement actually took care of your issue, but your issue is nearly identical to mine.
Any help would be greatly appreciated!!

Thank you in advance,

Ian
290794.jpg
Supra
 
Check all of your fuel lines a

Check all of your fuel lines and filters leading from the tank to the fuel pump. I had a similar experience and found a loose clamp at the fuel filter. It as sucking air at higher speeds. Tightened the clamp and all was OK.
 
"Hello Ian,
I have ordered th

"Hello Ian,
I have ordered the fuelpump but this is in Europe not so easy to come by! (It has to come from the USA)so i dont know for 100% sure it will cure my problem but i think it will.
See my previous mail
As soon as i have it installed i will post a new message.
I used a (temporary) clear tube between the pump and carb. and found my problem (air bubbles) mayby you can try this.., if this is also the case then find out if its the pump or what Roberts says "the supply to the pump".
Did the problem started after the short-out?"
 
I have placed the new fuelpump

I have placed the new fuelpump but the problem
is still not gone.
It started out ok but it looks like the problem has returned.
Also the other engine has the similar problem now.... Im getting crazy about it.
Maybe it does have something to do with the fuelsupply. Going to look into that further.
There are some strange non marine automatic pressure valves mounted for fueltank airintake/vent. maybe these are causing the problem.(by not working creating a vacuum in the fueltanks??) Going to swap the with normal manual valves.
 
Dick:

Put the pressure gaug

Dick:

Put the pressure gauge back on before you mess with those valves...what you saw as air bubbles may not be from an air leak. Having the gauges on will provided conclusive evidence that your problem is really going away.

May be quicker to just open the fill cap to relieve any negative pressure.

Sounds like these automatic valves are emission related and live in the vent line....are these common over there?
 
"Hello Dick,
I had a very sim

"Hello Dick,
I had a very similar problem quite some time ago. After spending too much time and money on diagnostics, I came to find out that the interior lining of the composite hoses from the exhaust manifold had deteriorated and were restricting the exhaust flow. They were not completely blocked so there was enough pressure at low idle to keep the engine running. As soon as rpm increased, the engine would sputter and often die. Effect is similar to the old "banana in the tail pipe". I would check your exhaust manifold hoses."
 
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