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Missing RPMs

B

bushwood

Contributing Member
"A familiar discussion as I re

"A familiar discussion as I read through different posts but hoping I am missing something...
M440 will not get past 3000rpm underway
-Timing is dead on; no back firing
-Advance mechanism is free
-Fuel pump has good pressure. Checked after last filter before going into carb
-Carb is good; in neutral, engine will race past 4000 (brief race to make sure she'll do it)
-Plugs clean, tan color
-bottom is clean, props were trued
-Comp test shows 7 cylinders at 145 and 1 at 125
-Valves are a bit oversized after valve job, but very close to spec

I am hesitant to pay a diver to pull props just yet, as engines used to get to 4000 (before heads done)with these props (15x18).

Anything else I can check mechanically before I venture down a different road?"
 
Sounds similiar to a problem I

Sounds similiar to a problem I had with my 440. I changed all 3 filters in the line and haven't had that problem since. 2 in line and Cermaic on at the carb.
 
"Did the valve job include spr

"Did the valve job include springs, etc.? If so, and you have the specs, you may want to confirm that they're appropriate for your cam grind.

It doesn't sound like the new valves are a lot bigger but...larger valves (intake) are better for higher rpm power production. Depending on gearing and load, engines tweaked for higher rpm power production can fall out of their torque curve before reaching optimum HP rpms. In other words, they've lost the low-end needed to get them to the high-end....so to speak.

I'm not jumping to a conclusion about the heads, but the valve job seems to be a variable and the fact that it'll pull 4k in neutral would indicate that it's a load-related issue.

Might want to check/confirm the specs on the valve job....to rule it out if nothing else."
 
"C: I lost 300 RPM from the t

"C: I lost 300 RPM from the top end. Changed all fuel filters. No improvement. Checked the spark arrester. It was full of lent like a clothes dryer. (How the heck did that get in there) I cleaned it out and found the lost 300 RPM. I realized the engine cover is lined with carpet on the inside. It built up over time.
Just a thought.
Good Luck, G."
 
Thanks for the input so far. I

Thanks for the input so far. I am going to change filters again. I think there is only 20 hours since last changed (beginning of season). Flame arrestor was clean (wife found out about dishwasher method-not happy). Secondaries may be a thought. I don't venture into the secondaries so I'm wondering if they are gummed up. Usually travel pretty slow. Any other thoughts?
 
"PLease tell me what you mean

"PLease tell me what you mean about oversize valves?
Is it the same set of heads?
Pull the spark arrestor off and make sure the throttle is opening all the way. (AFB?) If so, the secondary linkage is mechanical.
Bigger valves shouldn't make much of a power band difference, at 3K with a 440, with the same cam.

Here's another thing, if you pull the rocker shafts off, are all the valves at the same height? Run a straight edge accross them before you change the props. If some are sunk in, you may be floating lifters.

I'm putting my money on misadjusted throttle linkage."
 
"Linkage opens all the way. Ye

"Linkage opens all the way. Yes, heads are the same and when I say valves are 'oversized' they aren't performance, just a bit oversized."
 
"I wonder if your valves are t

"I wonder if your valves are too tight, do you know how to measure pre-load?


I have never heard of oversized valves, I mean, I have, they make bigger valves, but they are not like rings, where you get .010 over, LOL. Since they are different valves, I wonder if the stems are longer, giving you too much pre-load on the lifters, that would make the valves hang open at higher RPM, but still have good compression."
 
"Although the valves may not b

"Although the valves may not be related to the issue you're having, my curiosity has the best of me.

Typically, when applying to valves, "oversized" would relate to the head diameter. If valves that are larger than the original head casting are installed, the seats need to be machined accordingly. If the seat isn't opened, a larger valve could cause shorter lift and duration because it's seated too "high" on the seat. This could result in the tightness/pre-load issues that Bobby suggested.

Do you know if the seats were machined for the oversized valves?"
 
"Normally when an engine rebui

"Normally when an engine rebuilder refers to over size valves he means valve stems.
replacing worn valve guides is costly and time consuming, so they ream the guides oversize as needed and install oversize valves.
They are available in standard, .005/.015/.030 OS stem diameters.
you may have anywhere from one to all 16 reworked in this manner.
Joe"
 
"Over sized stems are a differ

"Over sized stems are a different story..... I never have used them, I install bronze guides.

That said, I am still wondering if your lifters are bottomed out, and the RPM's are pumping them up and hanging the valve open slightly. Seen it done before. If the vlaves were too high in the seat, that would cause it to clatter, and have less duration, but you would hear it, for sure."
 
may want too check the timing

may want too check the timing at elevated RPM. I've seen "free turning" advance mechanisms that bound up and wouldn't let the timing go all the way up. Pretty quick and easy if you have the light.
 
"Get a digital timing light an

"Get a digital timing light and check the timing at 3000 rpm. Should be minimum 28*. We had twin 440's that would not go over 3000. Found out that when they rebuilt the distributors they put the stiffer springs in because of the quality of gas these days. When they put the softer springs in, the problem was solved."
 
"Thanks Don. The mechanic tha

"Thanks Don. The mechanic that went over the engine had a dig timing light and said it was advancing all the way up to where it should be. He suggested to look at reluctor and make sure it was in correct position. At my untrained eye, it looks like it should-per the 'Service Bulletin.' My secondaries are also opening up-carb is clean and free. I guess next is to pull props and go that route. Will keep you posted!"
 
you said earlier you only have

you said earlier you only have 20 hours on since the beginning of the season; how old is the fuel in the tank?
 
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