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Old 05-31-2008, 11:21 AM
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Hard starting Tri Power

Got a 350 Chevy With Vintage Speed 3-Rochester's. Hooked up a vacuum guage to center port just below throttle plate, got no vacuum at cranking and only 5 " at idle. max out at 10 " at any rpm always hard start... Should I be using the only other vacuum port in manifold for testing?? it is a 1/4" port that I have never used. Also the throttle base seems to be leaking vacuum at the bolt holes on the center carb, could that cause the hard starting?

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Old 05-31-2008, 01:17 PM
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I'd check vacuum at a manifold plug if you have one.

A leak would have to be HUGE to cause hard starting, Small leaks will make it hard to ever get a stable idle.

So many things to check for leaks;

- I'd start with warm idle, spray a TINY bit of carb cleaner near the throat of each END carb. If those outboard carbs have throttle plates installed even slightly "off center", or have tight linkage, then they will cause a leak inside the bases. If the idle jumps WAY up with a tiny amount of cleaner, you need to see if the linkage is adjusted correctly, or if those plates are not sealing. If you spray tons in there, it will affect the idle because it will find a way to dribble into the intake....and cause a misdiagnosis.

I'd also not forget all the normal tune up stuff when chasing a hard start...could be something as simple as not having full 12v to the coil during cranking.
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Old 05-31-2008, 02:28 PM
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Please define "hard starting"





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Old 05-31-2008, 06:17 PM
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tri power

Thanks for the insight, I checked the seating of the outboard carbs before I installed the tri power and they looked great, I have strong pull-back springs and I'm sure they are closed. I pulled the center carb to check the throttle base and was surprised to see two 1/16" hole just above each throttle plate. The holes go through the base casting to the outside air. I don't know what they are for but I guess it should'nt effect the vacuum until the plates open a bit. I'll put the whole thing back together and check the vacuum in the manifold, and also do the vacuum tests on the outboards.
The other question about the hard start is: Mostly hard to start when cold, after it warms up seems to start ok, but if it dosen't then it seems to flood easily and takes a lot of cranking with no gas to get it going. I haven't played with the float level, not sure what direction to go to stop flooding.
I have tried different method on the cold hard start, like no gas... then a little gas...then pump the throttle and hold open, thats went it floods, then I do no gas and crank for a long time and finally if I'm lucky it coughs and slowly starts. Any clues there??
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Old 05-31-2008, 06:42 PM
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How much Vacuum is it pulling?

Checked the timing?

Checked the vacuum advance?





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Old 05-31-2008, 07:34 PM
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Quote:
was surprised to see two 1/16" hole just above each throttle plate. The holes go through the base casting to the outside air. I don't know what they are for but I guess it should'nt effect the vacuum until the plates open a bit.
That shouldn't be that way. That would cause a partial lean condition at throttle barely open, plus it is unfiltered air going into the engine. Ok,I have no idea if you will get insulted by this question... are those 2 holes "to outside air", actually the threaded holes for the 2 idle mixture screws??? Are the mixture screws missing?? I had to ask, as I can't visualize 2 other holes on a Rochester 2bbl. I know you said they were above the plate, so I doubt it


Check cold start functions on the center carb: Motor off, partially open the throttle and you should see the choke flap close completely. Then with motor STILL off, go to full throttle. Now at full, the "choke unloader" linkage "should" partially open the choke flap. That needs to work in case you do flood the engine. Now start the car, and see if the vacuum "choke pull" opens the choke flap to a certain opening. (there are specs to measure the gap between carb wall & flap) . That choke needs to "pull off" as soon as it starts, so it won't flood out, or run wicked rich & barely idle.

If this car has a nornal ignition coil or a HEI, I'd also run a test jumper wire from the battery+ to the feed+ on the coil or HEI. This would be to verify that the ignition IS getting enough unresisted voltage during cranking(if there is no change)
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Old 05-31-2008, 07:57 PM
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To F&J

I know you think I'm nuts but the hole are factory... The jets are on the opposite side of the base, looking at the base from the side that dose not have the float or jets you see in the center a large vacuum port in the 1" tall base, I think its 3/8" pipe threads, just to the left and right are the holes that go through the casting just above the throttle plates. the holes are inline with the bolts that hold the base to the manifold. The base is painted black and looks like factory paint and the holes were put in there before the paint was applied. I was going to call Charlie at vintage Speed to find out why the holes but he is on vacation until June something.
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Old 05-31-2008, 08:05 PM
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moving this to the engine forum.
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Old 05-31-2008, 08:52 PM
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To slowride66

My vacuum is 0 on cranking, and 5" on idle, the max is about 10" anything above idle, if I punch it it goes flat for a few seconds...
I have moved the timing all over the place and currently have it at about 38 degree total advance. I don't have a vacuum advance dist. I have changed the springs and cams to have it all in at about 2500 rpm, it seems to run the best there, but always the hard cold start. I have a very mild cam. I did a compression test awhile back and all cylinders check out in the same range.
I think that would eliminate a timing chain slip or stuck valve. I have a problem pulling my dist. all the way out because of a recessed firewall. (have to take the intake manifold out) I can pull it up enough to move the timing, but the last time I did it was ok were it was. changed the plugs and gaps several times, plugs always look good. I run MSD ignition and it all checks out ok, great spark. At wits end...Keep thinking about the low vacuum. I will try to put it together in a few days and do more testing vacuum from a fitting I have on the manifold. If the vacuum is in the teen's then i can start looking some where else.
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Old 06-01-2008, 10:05 AM
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I am thinking you might not have the correct carbs for the secondaries.

I am by no means close to a expert or even a pert

But I do know the secondaries are just dumpers no idle jets .

Also I think you might have a Vacuum problem from a leak .

Do you have a stock intake & carb you can throw on there to eliminate some issues?

I mean to say a SB chebbie just always wants to run just a small starter bump & VAROOM its going.

I would like to see some pixs of the intake & carbs this might spark a memory in the dusty recesses of my brain box.

Now not being a "Pert" the 2ndaries should be considerably larger then the primary{if its the same as Pontiac}





If memory serves me there should be no idle screws on the 2ndaries

Do you have the linkage set up correctly? progressive?
Zero to 1/2 throttle primary 1/2 throttle to WOT primary & 2ndaries





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Old 06-01-2008, 12:50 PM
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To slowride66

I'm no expert not even close, I purchased the complete setup from Vintage Speed, and I'm trusting them...the outboards are not jetted and they seem to be sealing good. the center carb is ported at the base below the throttle plates and Im using that for my Brake booster, pcv, and a/c controls, thats the only vacuum that I have (my dist.is mech. with no vac. adv.) I'm thinking of using a 1/4" vac. port that at present is plugged on the manifold, going to assemble today and test vacuum from there. also going to readjust elect. choke I may have been over choking on start-up causing flooding and hard start. Thanks for the tips
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Old 06-01-2008, 04:27 PM
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I was just thinking... ... you did say it does idle?


I recalled a thread here a week or 2 ago where a guy put on a tripower manifold on a sbc. It ended up that the 2 thick rubber end seals were causing the head-mating gaskets to not seal. That would be a huge vac leak, but it also would not be able to idle.
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Old 06-01-2008, 06:16 PM
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To F&J

I've never had a problem with idle, I have a concern about the way Vintage Speed installed the carbs.. when I took off the center carb the gasket was stuck to the throttle base with sealant but not a bit on the side facing the manifold. I'm not sure if I should put it together with sealant or not, seems like there could be a vacuum leak if the surface is not completely flat. I assume the other carbs are done the same so when I figure out whether or not to use sealant I'll do all three the same. All my sealants silicone etc. say they are not meant for extended use with gasoline. Until I can get to the parts store to check out other sealants for carbs I'll wait...
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Old 06-02-2008, 01:30 PM
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It seems to me your vacuum is very low . . . . and the car is hard starting. I think this could be a skipped tooth on the timing chain/gear arrangement. I realize there is an issue with what seem to be extra holes above the plates in the primary carb. . . . . but that should not affect idle much if they are above the plates. But you are idling at 5"Hg. I'd find TDC of #1, then run a check with a degree wheel. Or, maybe you've just got a huge cam . . . .?
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Old 06-02-2008, 03:46 PM
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I dont think you should need sealer on carb gaskets.
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