I jsut got some jets for my Eddy 600 cfm carb to try to tune it a bit. I put .095 jets on there as opposed to the .098 standard size, since I'm at 3000ft. It runs great accelerating from low end and in the high end, about the same takin off in the mid-range. BUT, it has a flat spot at part throttle I can't figure out. About 1/6th of WOT. Can I fix this or am I going to have to just deal with it? I rarely accelerate that lightly anyway.

 

[&:] bob since your messing with an aftermarket carb ill assume were working on an older motor so i have to ask what kind of distributor are you using and do you know the difference between ported and manifold vacuum. the reason i ask is your symptom sounds as though it may be timing related but not sure with so few details. enlighten and get back to us kurt

 

I will if this dang thing will post!!
Procomp HEI distributer, on full manifold vacuum. I haven't reid it on the time vac since it started, I'll give that a shot. I've not had issues with the distributer before, and the timing is at 10*.
The motor bay had a bad exhaust leak which I fixed today and it helped a bit. It wont die when I accelerate from a stop, but it does still hesitate. I tried adjusting the accel pump and it cut the hesitation in half but it's still enough to be noticed and lurch a little bit. I'm thinking I may have to try a different set of metering rods.

 

[&:]bob lets change a few things first and see if it fixes your problem as it would appear as though you are having the problem i thought you might. first lets remove the vacuum from your manifold source and hook it up to a ported source(correct spot, should be one of the two ports on the carb NOT on the manifold). then re time your engine for 6-8 degrees at idle and 36 total if you have an adjustable timing light, and then see how she runs. i suspect this should cure all if not most of your symptoms although you may still have some tuning to do(fuel mix, advance curve etc.) if you dont know which is the ported vacuum port let me know and ill direct you. your problem is youve got timing that is coming and going at the wrong times kurt

 

Something to consider, vacuum for the distributor advance was moved from full manifold to ported vacuum (above the carb. throttle plates) many, many years ago for emissions reasons. Because of that, most people believe that ported vacuum is what the distributor is supposed to get connected to. I suggest you give this a good read. Read the whole thing, twice if you have to. And pay particular attention to the portion that's about 3/4 down the page titled "Timing and Vacuum Advance 101". Then mess with the engine.

 

The advance is on the carb., but on the full vacuum port. It does have timed and full vacuum, I'm going to go and change em off once I get offline here, and I'll adjust the timing down to 8* (which should be stock specs if I'm not mistaken) I don't have an adjustable timing light, my dad ended up buying the cheapest one he could find, and I can't afford to buy one.I will adjust it to 8* at the very least and report back ASAP.

 

There we go, perfect! Timed vacuum port on the carburetor fixed it, thanks a million guys![8D]

 

camaro 69 i think you have your timing info backwards unless ive recently become brain dead. the vacuum pot on a distributor (with the more recent emission ones being the exception) has always advanced the timing and as such would have to use a ported source as a manifold source would provide advance at idle (not where you need it for power production or more as it was intended for mileage) as well as producing the symptom we fixed here. the emissions ones we mentioned were timed using different procedures and specs to produce better emissions. vacuum and electrics were used to modify the timing but still did the same basic thing and that is to advance the timing as load and rpm increase kurt

 

Using ported vacuum isn't always a carved-in-stone procedure. Some engines, especially heavily cammed ones, will actually idle better if hooked up to manifold vacuum. You might want to do it that way as opposed to bumping the initial timing up more, which would then affect your overall timing (making it too high). Of course, you also need to tune your distributor accordingly with the proper vacuum advance unit, or an adjustable one. So if you needed to, you could hook up to manifold vacuum to give a good idle, and then have the centrifugal weights in the distributor take over as the vacuum starts to drop when the throttle plates open. Also, having the distributor hooked up to manifold vacuum can sometimes alleviate at-idle engine overheating problems.
So, that's why I gave that info, since there are always exceptions to every rule. It gives you alternatives to experiment with. If one vacuum source doesn't work well, try the other.

 

[:@] how many heavily cammed engines use vacuum advance?? most really cant as they have very low vacuum at idle and rely on mechanical for all thier timing. ported is only good for off idle and a little more. much more than idle and mechanical starts to kick in and makes ported useless, ported vacuum goes away at this point anyway as well as manifold vacuum. im no race engine expert but ive served my time and built stock to fairly radical engines and NEVER seen myself or anyone i know, and i know lots o gear heads and certified techs like myself, use timing in the manner you describe. the 80's emissions versions being the exception. how is your hot rod set up, all mechanical ill bet!! stock and mild to moderate engines use ported for drivability and mileage because as you know vacuum continues to diminish as load increases. for the more radical stuff only mechanical will work for several reasons.

lets look at your scenario in order to have your car idle you can not have too much advance( 10-15 and running becomes questionable). so lets use 6, now in order to set your idle speed timing at 6 with the manifold vacuum hooked up one must retard the base timing by 10+- degrees to around 5-10 atdc (they dont like to run here either) this does two things, it drops timing when you need it most as the vacuum goes away with increased load and in most situations it also limits total mechanical advance which then limits power production. i just fail to see how this could work in any positive form, may be able to make it null or have negative effects?? if you want to say it works in cammed or modified motors well you cant really say that as vacuum is at a premium or non existent and stall converters make vacuum advance useless. have you studied advance curves?? mechanical usually starts to kick in 1000-2000 rpm for those that use vacuum and those that dont and its at this point vacuum advance becomes worthless. cammed motors dont start to come alive until this point so not only do you not want advance before this but mechanical will be the only way to get it.

as for overheating well if this helps you have other problems as you cant retard or advance it enough at idle and still have it run to produce enough heat that a properly functioning system isnt easily be able to get rid of that heat. try it see what happens!! `if tuned properly WOT will produce the most heat. top fuel how long can they idle, quite awhile. now how long do they run at WOT thats right a couple seconds. i know they dont have coolant but heat generation is the same. they can also mess with mixture a little but idle is when the least heat is being generated.

look forward to your reply as i enjoy keeping all this straight in my head and love to help others understand kurt