Inliners International Forums General Discussion Engines Engine Tuning

My 235 engine will not run properly; when I shift from 2nd gear to 3rd gear (standard original 4-speed from a 1951 1/2 ton truck.) the truck starts off very well but after shifting from 2nd to 3rd gear the truck stumbles and has no power .

The engine is a rebuilt 1954 235 engine with Fenton dual exhaust and dual Rochester's on a Fenton intake. Both carbs have been rebuilt with matching components and they have performed well for over 2 years. The carb jets have been reduced from 51s to 49s due to running too rich. Both carbs have been balanced using a UniSync and the engine easily idles at 700 rpm but I have it set at 900 rpm. the engine cam is not a stock cam for the 235,it is a standard 261 cam without any changes and the springs are from an S54 Bulldog cam.

The only thing I've changed is the cam; and the front oil shaft the rocker arms pivot on (the shaft broke in half at the center bolt hole). Before the cam change from a Bulldog cam to the 261 cam the engine ran very well in 3rd and 4th gears. Changing the jets significantly reduced the "too rich" problem with no change in using "2nd" gear to start moving the truck.

I just don't know what it could be, all help is appreciated.

With the cam change you may need to change the timing. If you disturbed the Intake check for a vacuum leak at the head and carbs. I don't know how different those cams are but the mechanical or vacuum advance may be wrong now. Check full throttle in both carbs. Too lean now? Good luck I've been chasing an ignition problem for a month.

Im surprised the 261 stock cam needs to be at 700 to idle smoothly. and setting it to 900 seems excessive to me. I just finished my new cam install a few days ago and its a 251/254 cam and i only need 600 rpm to be smooth at idle.

Why did you switch from the bulldog?

couldn't it be your clutch slipping, if in first everything is ok but second its nearly not moving i would suspect the clutch instead of the engine.
Or might be a vac leak aswell.

Good morning, I had not thought about a vacuum leak, I will be sure to check that out. the issue isn't the clutch, although I had not thought of that either. it's like I've just got past the off-idle stage and there isn't enough fuel provided to keep it running; it bucks and stutters like it's running out of gas.

The other side of the coin is with the 51 jet it was obvious the engine was running too rich, won't the increase in jet size cause the carbs to be even richer?

The engine runs smoothly at 650-700 but I'm just being careful, I'll get it down to where it needs to be after I get the rest smoothed out. watching the tachometer I'm seeing a slight swing in rpm, maybe 50 rpm or so, 700 to 750, etc. I used to have a Petronix pointless ignition (replaced the points, etc) and I got a continuous surge and return of about 6 degrees that drove me crazy so I took it out. A friend of mine likes his in a stock, single barrel 235.

I'll check full throttle in both carbs (disconnect one and try them separately?) and I feel good about the vacuum advance; it responds immediately and without hesitation.

as for the Bulldog cam I never felt like I got the street response I expected, perhaps I never got the cam properly set up. Any one interested in a Delta S-54 cam with an aluminum gear still attached to the end?

My understanding is the stock 261 cam was installed in the original Corvette 235, that it is relatively similar with a small difference in duration (longer) and as I just finished building a 261 with a Bulldog cam (yes, I'm a slow learner) I just happened to have a 261 cam with the gear all ready attached.

I agree I may have the carbs jetted too lean. I dropped the jets down from 51 to 49 and the stuttering got worse. I spoke to Patrick and he told me to go back up, to start at a 56 and go from there. I hadn't thought about it, but I've been gathering up old Rochesters for years and my collection of 56 jets is at a ration of 3 to one. the next step will be verifying no vacuum leak and a jet change to 56.

What about valve lash settings? I've all ready reset them with the engine hot.

Anyway, the saga continues, thanks guys, I'll let you know the results.

It sounds like you are making progress. At least you have some Ideas to keep you busy. When I suggested to check the full throttle it was just to make sure both carbs are at the same place. Both fully closed and both wide open, just a visual check. If it is too lean it will stumble while moderately accelerating and maybe cruising. You may also need to set the accelerator pumps a little richer. Keep us up on what you figure out.

Have you tried to sync the carbs using a sync tool? Also if you hook a vac gauge to your intake what is the gauge doing? holding steady? bouncing?

Hi, here we go again, I'm going to respond to all of the questions/suggestions listed above.

the clutch does not slip. The sync tool has been used successfully. The vacuum gauge holds steady at 14.75.

How do I set the accelerator pumps a little richer?

There is also a piston and spring assembly next to the tower where the jet is installed. How is that spring adjusted to modify the operation of the carburetor?

I used ETHER to check for vacuum leaks; we tested around the three intake connections (intake to engine, carburetor to intake, sides of carbs for butterfly valve shafts being loose), with no issues at all.

Timing is interesting. We set the timing up at about 8 degrees and found out it runs best at about 12 degrees. Then not using the timing light we rotated the carb counter-clockwise and then clockwise looking for what the engine liked the best and set it there; approximately 12-14 degrees.

i ran the carbs from idle to full throttle and they both respond the same way at the same time. Near full throttle we get a popping sound, no longer a steady roar. the carbs respond immediately and without hesitation.

I installed a used 261 cam and so am going to run a compression test next to make sure I've got proper valve operation. If It does not have good compression I'm taking the used cam out and will install a new 261 cam.

Thanks, for all the help guys,

Ted

Well it's time for an update. Not being able to come up with any other answer I did a compression check (got the engine hot, removed the plugs, kept the carb throats clear)and found #1 to be 120 and #2 to be 45. I stopped there and the next step is to buy a new 261 cam and a new set of lifters.

The problem cam came out of a 1954 large COE truck and I was told it was running when removed (that's why I installed it wanting to replicate the early 1950s Corvette engine (235 with 261 cam). Do any of you know what the specifications are to have a local shop re-grind the cam to original specifications for the 261 cam?

All help, comments appreciated,

Ted