454 P30 Chev Misfires When Hot Under Load

[rlbarkleyii]

Restoring a Holiday Rambler 1988 coach with 454 V8 in P30 chassis. Having a problem with the engine. When engine hot on the highway, when second venturi of carb is opened the engine seems to die momentarily. It soon recovers generally without stalling. This happens ofter when pulling up a rolling hill or accelerating briskly from stop. Engine electrical, distributor parts recently replaced. Runs great on flat road at minimal throttle.

Any wisdom I can get as to the cause and cure of this problem would be appreciated.

[Jon796]

Inside that carburetor is an "accelerator Pump". It is a diaphragm that pushes a little bit of extra fuel into the carburetor when you accelerate hard. It is VERY common for these to go bad over time. The good news is that they are cheap and easy to replace. Look on the carburetor for numbers that are stamped into it. Take them down to your local parts store and ask for an "accelerator repair kit".

[LivingLikeEachDayIsMyLast]

Inside that carburetor is an "accelerator Pump". It is a diaphragm that pushes a little bit of extra fuel into the carburetor when you accelerate hard. It is VERY common for these to go bad over time. The good news is that they are cheap and easy to replace. Look on the carburetor for numbers that are stamped into it. Take them down to your local parts store and ask for an "accelerator repair kit".

The accelerator pump only works for the primaries, not the secondaries.

[LivingLikeEachDayIsMyLast]

If your carb is a Rochester Quadrajet & bogs when the secondaries open, the secondaries are opening too fast. It's also known that old spark plugs & plug wires can cause the problem you are experiencing.

[MikeCanter]

For your hesitation or bog when the secondaries open go to the website and scroll down to "Setting the Air Valve Spring Tension" http://www.73-87.com/7387garage/drivetrain/myqjet.htm

That site has pictures to show you where the little adjustment screw is. The hard part is undoing the hex head lock screw on the bottom of the adjustment screw. You can do this with the carb on the motor. The pictures show you the location of the hex head screw.

One real bad thing about that carb is that if the floats get old they get saturated with gasoline so they sit too low in the bowl and the needle jet will not close so the motor will run real rich and the idle will be rough and rich. If the floats are saturated with the engine at idle you can go to the exhaust pipe and it will be rough and possible have black smoke coming out. If you take the aircleaner off the carb and look down the carb throats while at idle you can actually see drops of raw fuel dropping down against the butterflies. This rich mixture will load up your spark plugs real fast so if you floor it and accelerate hard then the motor will start misfiring and popping. If the plugs are old it will also misfire when floored. My big block 70 Chevelle was real bad at doing that.

[georgeschweikle]

Have you changed the "secret" fuel filter?. My 1983 454 Winnebago used to do the same thing until I discovered an in-line fuel filter hidden behind a frame rail halfway between the front & back wheels.

[dickandlois]

Many of the posts have covered the first level of items that could cause your problem.

Rapid Acceleration not only involves the accelerator pump, but opens up the primary and secondary’s. You also mentioned the issue also happens when climbing.

My thought(s) are.

1.. The engine is running rich due to dirt in the carburetor, secondary linkage miss adjusted ( as mentioned)

2. There could be one or more vacuum leaks; they mess up the fuel to air mixture.

3. Spark plugs (As mentioned along with the plug wires)

4. Distributor Cap and rotor.

5. Engine timing off.

You did not mention the number of miles on the engine, so it’s hard to say if the problem is internal. I did drive a Class A, 454 powered coach over 125,000 miles and it was traded for an AD unit. The only real issue with it was the valve lifters got noisy due to low oil pressure when idling.

I do not like noise of any kind, so the oil pump was replaced with an 80psi.HD unit.

That is another story for another day.

****

[hermanmullins]

Does it seem to hesitate when you accelerate?

If so you may have a bad accelerator pump in the Carb.

[loudecuz]

I know its been awhile since anyone has posted. Here's what I ran into as we were driving 1988 Winnebago 454 Rochester carb.

We were slowly losing power and if I gave it more gas it stumble and popped out the carb. It progressively got worse the more we drove. On the orad we replaced the filter inside the battery tray, the filter in the carb, cap rotor and electronic ignition module.

The secondaries were not opening so I had to adjust the vacuum screw to allow the tab to pull something on the right side of the carb which was blocking he secondaries from opening. MOST importantly the vacuum advance on the distributor was stuck the vacuum part was fine from sitting the actual part that advances was not turning. A few screw driver shoves some wd and it started to move when vacuum was applied you can do it with a vacuum tool or just use your mouth you will get gas fumes in your lungs but when stuck on the road it does not matter, The plugs were in great shape and the wires were dry rotted literally falling apart. I was luck I had wire covers which prevented any side spark.

After completing all of the above it ran like new!! I think the key was the advance was stuck in the distributor and the filter was clogged. The carb was super clean when I took it apart so its not the carb. Can't be the little accelerator pump-- that would not cause a stumble, This all worked for me I think I could have cut it to carb filter and advance in distributor. No one mentioned that for all the stories I read.

Thanks!

[dickandlois]

Loudecuz,

If you had to adjust the secondary pull off setting, I have a feeling that your secondary pull off vacuum Assembly is defective or you have a bad vacuum line to the assembly.

You did mention the mechanical / vacuum advance was stuck, instead of WD, use a better spray lub. or a light oil to keep it free longer and reduce the chance of rust build up.

Be very aware of vacuum lines that have not been replaced on your engine, they get very dry and crack over time in the hot conditions under the Dog House.

Rich.

[kaypsmith]

I had 454 in p30 chassis 1988 Southwind Class A, only thing that worked for me with similar issues, was to go to a racing store, purchase a new solid center plug wires. I tried three different sets of the over the counter sets with carbon center, none of them worked for any length of time. The racing version worked for me three years, and were going strong when I sold her.

This may not be your issue, but a thought to remember.

Kay

[ticat900]

Many of the posts have covered the first level of items that could cause your problem.

Rapid Acceleration not only involves the accelerator pump, but opens up the primary and secondary’s. You also mentioned the issue also happens when climbing.

My thought(s) are.

1.. The engine is running rich due to dirt in the carburetor, secondary linkage miss adjusted ( as mentioned)

2. There could be one or more vacuum leaks; they mess up the fuel to air mixture.

3. Spark plugs (As mentioned along with the plug wires)

4. Distributor Cap and rotor.

5. Engine timing off.

You did not mention the number of miles on the engine, so it’s hard to say if the problem is internal. I did drive a Class A, 454 powered coach over 125,000 miles and it was traded for an AD unit. The only real issue with it was the valve lifters got noisy due to low oil pressure when idling.

I do not like noise of any kind, so the oil pump was replaced with an 80psi.HD unit.

That is another story for another day.

****

Engines do not run rich due to "dirt" in the carburetor and if it was running rich this would not cause a back fire while hot and under load

99% of the time a backfire while hot and under load will be a bad plug wire or a shorting to ground plug wire(burnt though or bad boot).

[ticat900]

Restoring a Holiday Rambler 1988 coach with 454 V8 in P30 chassis. Having a problem with the engine. When engine hot on the highway, when second venturi of carb is opened the engine seems to die momentarily. It soon recovers generally without stalling. This happens ofter when pulling up a rolling hill or accelerating briskly from stop. Engine electrical, distributor parts recently replaced. Runs great on flat road at minimal throttle.

Any wisdom I can get as to the cause and cure of this problem would be appreciated.

Your Subject Heading about backfire and your post indicating about a bad bog or hesitation.

You are complaining about 2 different problems here?

[dickandlois]

ticat900,

Dirt does build up in the carburetor float bowls over time and if the carb. sets and the fuel evaporates these deposits dry out, then mix with the fuel and can get caught between the needle and valve seat, causing a continuous flow of fuel that can be from a minor to a major amount of excess fuel being delivered to the engine.

A poor seal between the needle and seat caused by the seal drying out can cause the same problem.

The Rochester Q-jet model E4ME came in to styles, one with a MC valve and one without the valve. The difference being the weight class of the vehicle and the EPA requirements for the class of vehicle. There is also the E4MEU series.

These Dancing needles as they are referred to, need to be set to the proper dwell, generally 30 degrees and this cycle timing is controlled by the ECM by comparing a given voltage reference and the voltage being sent to the ECM from the O2 sensor that measure the exhaust temperature to maintain a good drive ability curve for the engines.

One can get the book Rochester Carburetors by Doug Row that covers them quite well, what is missing is the information on the importance of the dwell settings and for the most part are only in the shop manuals for each year vehicle.

The MC Solenoid is pictured in the Book mentioned, but the set up of the solenoid and how that setup affects driveability are not covered.

Two years ago I was fortunate to have a conversation with one of the primary engineering technicians that developed the fixtures and the setup of the system and a copy of his notes.

The late models of the Q-jet carburetor can be a nightmare to setup after a rebuild because of the narrow window the MC Solenoid needs to be set at.

The shops that can repair and setup these carburetors are somewhat limited. There is the Sun Cruiser Chapter in FMCA that very proudly repair and restore these Coaches built by GM and are powered by the Oldsmobile Toronado Engine and front wheel drive trains.

This Chapter might be a source of information for the different Q-jet setups.

The High Energy ignition system and its operation are another story.

Rich.

[ticat900]

Rich,

I was a very successful master mechanic for 40+ years.I know how q-jets work and their little quirks. I fixed them by the hundreds

the electric fuel control ones were not that common on cars and light trucks.

Then in 1988, the TBI systems started. However the OP problem in my opinion is more than likely bad plug wire or shorted plug wire or bad boot

On a side note I use to throw away the late qjets and install a carter 625AFB or a spread bore holly.

I must add that I only did this on ESC systems.I never did a ECM controlled fuel valve Qjet system.

[Poppo]

It's amazing to me the amount of information contained in these threads. Hi five to the contributors! Where were you guys when I had my first MH, a 33' 1983 Georgie Boy Excalibur with a Chevy 454 that would always hesitate when I climbed a hill and the dog house was hot ... I was always told it was a vapor lock.

Hmmm ... 1984 ... that's right, Al Gore had not yet invented the internet.

Seriously, the information on these FMCA forums is invaluable, thank you for sharing your experiences and talents.

Poppo

[billbaldwin]

I have the same engine in my dually. This is exactly what mine was doing checked all of the above, was a flat cam.

Ran smooth at idle, stalled a bit on acceleration and ran great at hi-way speed.

bill

[ticat900]

I have the same engine in my dually. This is exactly what mine was doing checked all of the above, was a flat cam.

Ran smooth at idle, stalled a bit on acceleration and ran great at hi-way speed.

bill

A flat cam on a intake lobe will make it rapid steady backfire(actually blows back through carb)under heavy throttle especially under load but will do so when hot or cold.A flat lobe on the exhaust side one will notice little drivability problems.

OP says :454 P30 Chev Misfires When Hot Under Load I don't think what OP is describing is what a bad intake lobe acts like.