Tuning a Tri-Carb

[tsmith8]

I am new to the ti-carb setup.

Using three two barrel rochester carbs with a progressive linkage. This is on an Offenhauser intake on a 327.

I do not know much about this setup. I want to give it a good tune up and any input would be appreciated.

[tmoble]

do the secondary carbs have idle mix screws in the baseplate? If no back out the idle speed screw till the carbs are completely closed. Then, adjust the center as if it were the only one.

for best all around results, adjust the linkage so the secondary carb opening is delayed as long as possible.

[tsmith8]

Originally Posted by tmoble

do the secondary carbs have idle mix screws in the baseplate? If no back out the idle speed screw till the carbs are completely closed. Then, adjust the center as if it were the only one.

for best all around results, adjust the linkage so the secondary carb opening is delayed as long as possible.

Only the centre carb has the adjustment screw.

[tmoble]

still working on this?

[tsmith8]

Yes I am.

[tmoble]

have you got it running yet?

Is this a new setup? or an old setup being resurrected?

has it ever run on an engine since you've had it?

[MiraclePieCo]

I've done several tri-power setups. Why do you want to "tune" it - do you have a specific problem? Tri-power's biggest enemy is vacuum leaks; this can be identified by surging at idle. Make sure your throttle plates seal perfectly on the secondary carbs. Next biggest problem is bogging when transitioning to secondaries. If it's doing that you need a bigger accelerator pump shot and/or larger primary jets. Idle screws are hard to reach on the center carb; they may special flexible screwdrivers especially for this purpose. Generally just turn them out until highest idle rpm is reached, then back in 1/4 turn.

Here are pics of some of my previous units, all converted from stock 2G Rochesters:

[sweep]

I have the tri carb setup and as it idles and mostly runs on the center carb it's rock solid and easy to maintain. Tuning for open throttle is trickier and involves jetting.

[tsmith8]

I was having issues driving home Tuesday morning. Felt like it was running out of gas(full tank). It just didn't want to go. I bought this truck last fall and have only started driving it over the last month. It sat for four years in a garage before i bought it and the engine has about 20,000Km on it since built.

It drove fine Tuesday morning to town but was only having issues on the way home. It sat for just over an hour before driving home.

Figured a tune up was in order and trouble shoot a few other things. I took it for a short rip today and seems to be almost there.

[sweep]

1) Clean carbs if the truck has been sitting.
2) Is your fuel fresh?
3) Check fuel pressure/fuel pump

[AnotherWs6]

The FIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIRST thing I would check is your fuel filter. (I hope it has one!)

Then as sweep said, clean the carbs. But don't take anything apart or fiddle with anything until you are sure that you have good, clean, fresh fuel flowing to the carbs.

[tsmith8]

Will install new filter tonight. The gas tank was pulled and emptied in the winter. Installed back in and filled with all fresh fuel.

I will keep everyone posted.

A huge thank you to everyone, i love having this type of resource and feedback from everyone.

[DransportGarage]

Originally Posted by tsmith8

Will install new filter tonight. The gas tank was pulled and emptied in the winter. Installed back in and filled with all fresh fuel.

<snip>

Did you look in the tank? Was it clean? Did you pull the sender and inspect the "sock" on the pickup tube?

I've seen multiple fuel filters clog over and over because of debris in the tank. If getting answers to the above questions at this stage is tough to do, you might take your new fuel filter off, cut it open and do an autopsy on it.

[tmoble]

what they said, the filter is suspect if it sat for years. Sounds like you're already flushed it out and got fresh gas.

can you post a pic of your setup?

[tsmith8]

Have not changed filter yet. My son was sick last night.

But nailed down the timing today. I was off.

Now here is my issue. I can jump in the truck fire it and and take off down the road. Run through the gears 1,2,3 when i am in 4th at a certain point i hear what sounds like a rattle. If i shift into 5th i hear that same noise. When i am off the accelerator no noise. When i am on the gas i hear it and the truck doesn't want to speed up very fast.

When i am slowing down i can hear that noise sometimes. (issue not believed to be transmission related)

Any thoughts from anyone?

[tsmith8]

As requested.

[1project2many]

If the rattle sounds like marbles in a glass bottle then it's likely spark knock. It could be too much advance, insufficient octane level fuel, lean mixture, hot engine operation, carbon build up, or a combination of problems. How much do you know about the engine build? Cam, compression, etc?

[tsmith8]

Originally Posted by 1project2many

If the rattle sounds like marbles in a glass bottle then it's likely spark knock. It could be too much advance, insufficient octane level fuel, lean mixture, hot engine operation, carbon build up, or a combination of problems. How much do you know about the engine build? Cam, compression, etc?

I know nothing of the engine build happened several years ago before sitting for four years. This issue just started, it worked well for awhile after getting it on the road this summer.

[tsmith8]

Ok i have a build sheet for the engine and transmission. It has charts with HP and everything that went into it. I also had this sheet of paper attached to it.

Compression of the engine is 9.5-1

[1project2many]

Sounds like a mild build with no craziness. Cam looks to be a Comp Cams 252H. That 4.04" bore plus the small chamber powerpack heads could make compression a bit higher than expected so I wouldn't rule out knock. The sound of spark knock is unmistakeable once you know it but how to describe otherwise??

This video has some spark knock. Listening on a laptop it really doesn't have the higher tones that sound "snappy" to me, but it might give you an idea to say "close" or "way off." https://www.youtube.com/watch?v=rFuLwzb_VmE

You could also try turning the distributor back 3-4 degrees to see if the noise goes away. You said the timing was off... Did you happen to check where it was, or just reset it?

I don't want to lead you too far off the path in case the noise is something else. Maybe you can get a video with the sound happening?

[AnotherWs6]

Excessive carbon build up in the combustion chambers can casue knock. Something to think about since you said it was fine for a while and has recently developed the issue. You could have been loading the chambers up with carbon for a while, and now it's at the point where you are starting to hear it. A thought..... I don't want to lead you on a wild goose chase. But if you retard the timing and it helps the it's something to think about. Have you pulled a plug/s? How do they look?

[tsmith8]

Ok, made some big headway last night. For some reason the distributor was not giving advanced timing. We changed the springs on the weights as they appeared to be chasing the issues. GOOD news, it worked. Timing is set.

We adjusted the centre carb to what we believed to be the best setting. (using a vacuum gauge)

I pulled the truck out and took it for a drive. It is not chugging not wanting to go when the RPM's are believe 2000 +/-.

I don't know why, we are confident timing is 100% dead on. The centre carb we are confident we are very very close.

Now to try and figure out where to go from here. We have perfect timing and great fuel delivery. I believed i am going to check for vacuum leaks tonight.

[tsmith8]

I just got home from work, jumped in the truck and took it for a drive. Worked like a charm. Beauty.

Let it sit for another 20 minutes in garage and started it again idles perfect for about 2 mins. Idle started to fluctuate down and eventually stall out. Started it again and it was doing the same thing as the end of the night last night. If i go to give it throttle it chugs and stalls out. Have to try and feather the gas to get it to want to raise the RPM's.

I am lost. Any thoughts from anyone at all?

[1project2many]

I'd guess fuel related, but is it too much or too little? Did the exhaust smell burn your nose? Can you remove a plug and see if it's black or if it's clean? If you tried to open the throttle, did the engine backfire through the carb at all? Did you try to drive or keep engine rpm up to see if it got better?

[tsmith8]

Originally Posted by 1project2many

I'd guess fuel related, but is it too much or too little? Did the exhaust smell burn your nose? Can you remove a plug and see if it's black or if it's clean? If you tried to open the throttle, did the engine backfire through the carb at all? Did you try to drive or keep engine rpm up to see if it got better?

The exhaust did not burn the your nose.

I wil check a plug this afternoon. Did not back fire, i tried to keep the RPM's up and drive it as did my brother and it was a losing battle and would stall out.

When the engine is warmed up it would bog down and start to stall when giving it throttle and the idle was up and down by a few hundred RPM.

When off and throttle linkage moved it appeared to have a good feed of fuel in carburetor.