Carborator or timing issue?

bdepedro · Apr 16, 2012

I'm having some tuning issues and I can't figure out if it's a timing or carb adjustment issue. The car runs fine through the primary range and even when the secondaries open as long as it's gradual. If I get on it fairly hard I get a lot of pinging and smoke. I changed carbs last year and it's been doing it since. I've rejetted the primaries so the car is not running too rich or lean. I believe the timing is set to 14 DBTDC (need to double check this). I recently changed the secondary jets from 66 to 69 and haven't noticed a change. At this point I'm thinking of reducing the timing a few degrees or changing the secondary spring (both on suggestion from different friends who are more knowledgeable than me but don't seem to agree).

The engine is a 289 that has been decked 0.10. The heads are ported and polished 351W heads with roller rockers, double valve springs and stainless valves. It has 10.5:1 pistons, the cylinders are bored 0.30 and it's got a 0.512 lift/292 duration cam. I changed the carb from a Holley 670 street avenger to a 570 street avenger.

I know it's hard to diagnose a problem this way but I would be interested in knowing which avenue I should head down and I'm also wondering if the problem may be that the carb is not big enough for the engine mods. I appreciate any advice, thanks!!!

woodsnake · Apr 18, 2012

Where is your timing at right now? When you did your last tune, did you use a vacuum guage?

PoppyMod · Apr 18, 2012

Hi,
You best attack, is to add O2 sensor bungs to your headers. After which, visit your local speed shop with a chassis Dyno, remove the air cleaner, have them attached an AFR meter then make 3 runs from about 2500-2800 to your expected red-line.
Then review the AFR. Ideally, you want an fairly flat AFR of about 12.9-13.3-5, or so. It's OK if it goes slightly lean when they "punch-it", but then, it should level. Use this to decide what to do with both primaries and secondaries.
It's worth the $150-200 for the runs and advice.
Good Luck!

69Rcode_Mach1 · Apr 19, 2012

PoppyMod said:
Hi,
You best attack, is to add O2 sensor bungs to your headers. After which, visit your local speed shop with a chassis Dyno, remove the air cleaner, have them attached an AFR meter then make 3 runs from about 2500-2800 to your expected red-line.
Then review the AFR. Ideally, you want an fairly flat AFR of about 12.9-13.3-5, or so. It's OK if it goes slightly lean when they "punch-it", but then, it should level. Use this to decide what to do with both primaries and secondaries.
It's worth the $150-200 for the runs and advice.
Good Luck!

This is good advice, makes for a really good picture of what's going on. What is your total timing and what rpm is it all in at? Best approach is to do deal with it one at a time. Get the timing set correctly, then work on the carb.

PoppyMod · Apr 19, 2012

69Rcode_Mach1 said:
This is good advice, makes for a really good picture of what's going on. What is your total timing and what rpm is it all in at? Best approach is to do deal with it one at a time. Get the timing set correctly, then work on the carb.

Hi,
First, you need to determine the built-in mechanical of your distributor. This together with your initial, will be your total advance. As an example, I'm running a '67 dist. which has about 23 deg. built-in. I'm running about 14-15 initial. This makes for a total of 38-39 total. Most of our SBFs will tolerate a total of about 38 deg. However, there are any number of variables, fuel quality, ignition quality, combustion chamber shape, altitude, cam events, and power band, etc.
Also know, you have about 10 degrees of adjustable advance built in to your dist. vac canister.
Typically, you will want your total "all-in" by about 3000 Rs. Again, each engine and its components can cause this to vary. I've seen engines with "all in" by the mid twenties.
My engine build seems to run well at about 3K Rs. You might "creep" up on as much initial advance as your engine will tolerate and still start easily. Keep in mind the impact of your dist. vac will affect your mid-range and over all total without any detonation (too much advance and pinging).
Your engine's optimum settings can only be learned through testing on your local test ground or on a chassis dyno.
Good Luck!

woodsnake · Apr 19, 2012

BY chance, have you tried to run the engine again, with the previous carb? Or did you get rid of it?
" If I get on it fairly hard I get a lot of pinging and smoke. "
Pinging to me says too much advance. Are you saying that the smoke you get is fuel?
Yours seems like a not over the top build, similar to my 289, and either carb should work great for that application.

MitchGT · Apr 19, 2012

what color is the smoke? black, like unburnt fuel?

bdepedro · Apr 20, 2012

Thanks for all the responses. The nearest dyno is 1 1/2 hours away so I'll have to make some plans for that. Sounds like a smart way to get a precise tune though.

I'm running an MSD distributor, coil and controller so I'd have to see if I can figure out the built in advance. I'm going to check this weekend to see what the current timing setting is. I set it last fall but can't remember.

The smoke is grey and with the pinging, I'm thinking a very lean situation under load. Does that sound right?

bdepedro · Apr 20, 2012

I did get rid of the old carb but could easily borrow it or buy it back if needed.

woodsnake · Apr 20, 2012

If the car ran better before the carb change, and nothing else has been modified since, then it seems reasonable to find out what the jets are in the other carbs. Either one would be an acceptable size, the smaller one having better low end response.

bdepedro · Apr 23, 2012

I actually talked to the guy I gave the old carb to this weekend and he's going to let me check the jet sizes in it. It will be interesting to see what differences the jet sizes are to the carb I'm running now because the old primary system was rich and the current one is good whereas the old secondary system was good and the new one lean, or so I think. I'll post what I find out.

TommyK · Apr 24, 2012

The Avenger series carbs have notoriously lean calibrations and upsidedown fuel curves meaning they are rich at idle and lean at WOT. Having said that I think there is a possibility you have a combination of circumstances which are contributing to the issues you are experiencing which if left uncorrected could result in a serious boo boo to your engine. Unknown compression ratio, unknown total timing and lean WOT runs can equal "BOOM". When I ran a 670 on a 4V cleveland I needed an 8 jet split front to back AND had to enlarge the PVCR's to get enough fuel in it so it didn't lean miss under heavy load. Instead of paying for dyno time you may want to consider buying a wideband O2 system and learning to tune it yourself.

The quick fix is to verify your total timing isn't more than say 34 degrees and up the secondary jet a bunch.

Carborator or timing issue?

bdepedro

Member

woodsnake

PoppyMod

Member

69Rcode_Mach1

Active Member

PoppyMod

Member

woodsnake

MitchGT

Member

bdepedro

Member

bdepedro

Member

woodsnake

bdepedro

Member

TommyK

New Member

Similar threads

Carborator or timing issue?

Member

Member

Active Member

Member

Member

Member

Member

Member

New Member

Similar threads

Privacy & Transparency

Privacy & Transparency