Sorry for the double post..I mean't to put this on the tech board to begin with.

well...not quite thermal...but here's the facts..

73 coupe, 350-250 Crate motor. Still has original distributer and contact points which are new.

Timing was set at 8* initial, and vacuum advance was connected to a program port, which gave no measureable advance at idle when connected. With this configuration the motor ran ok with smooth WOT, but felt like something was restricting it. I changed the VA connection to a manifold port, and went through Lar's procedure for performance timing advance and ended up with the following:

14* Initial timing with VA disconnected
30* at idle with VA connected
36* total @ ~2500rpm

The car now runs great on the primary's..better responsiveness and the "something is hold us back" feeling is gone. But now when I go WOT, at about 4500rpm it starts to break down and backfire. The dwell is set at 30 on the points. I plan to start reversing these changes in the morning, starting with backing down on the initial timing advance. I suspect at this point that the improvement in primary throttle response is due to the change on the VA port, and that most likely is not contributing to the WOT breakdown.

Any thoughts before I begin tinkering in the morning?

The carb is a Edelbrock QJet model 1901 on a 2101 Performer intake. It seems to be working fine. No other adjustements have been made other than playing with the divorced choke which I now have working. I am thinking I may have too soft a spring in the dist which is causing the total advance to come in too soon?

thanks in advance for your opinions.
dosoctaves is online now Edit/Delete Message

 

Pull the vacuum advance off and plug it at the carb and take it for a spin, if it runs OK at WOT then the vacuum can is pulling too much advance

EDIT : Before anyone says there should be no advance at 4500RPM or no vacuum period yada yada yada, the suggestion is just a starting point to see how it runs without the vacuum advance adding any timing and muddying up the waters

 

At WOT you shouldn't be getting any vacuum advance, so I don't think that is a factor. The stock jetting on the 1901 is pretty lean - I have one, but I can't remember what the stock jets/rods are. I don't know if that is your problem but you should probably rejet (and rod) your carb for a richer mix.

 

The carburetor is leaning out as the secondary accelerator wells run empty. You'll need to go a couple steps richer on the secondary metering rods.

As this is an Edelbrock 'Jet, the primary is probably lean even though it feels ok. You can make the secondaries overly rich to compensate for this. It works, but it's not the right way to go about it. The correct way would be to open the carburetor up and tune the primaries but most people don't like to do that.

BigBlockk

Later.....

 

So BigBlockk, your logic is that with the timing adavnced, it's now starving for fuel when the secondary's are opened at high R's?

I am having a hard time understanding this as a carb problem when all i did was change the timing. But I certainly understand the theory...I think.

I think I might buy a holley 600 and try that. I at least I know how to tune them. Then i can pull the QJet and disect it at my liesure whilem I still have a running car.

thanks for the reply's and idea's. Please keep them coming..

 

With the timing advanced it now runs better than it did before. Now when it lays down at 4500 RPM you are able to feel it. Added advance will aggravate a lean condition, magnify it. The problem was always there, you just couldn't feel it.

Popping and backfire through the carburetor is a lean condition. At 4500 RPM the secondary air valve will be open somewhat (they should start opening somewhere around 3000 to 3500 RPM). When the secondary air valve first opens the secondary accelerator wells give the engine a small shot of fuel to cover up the lean spot that would occur. These wells continue to feed fuel well into the secondary opening but will eventually run low. When they run out, the secondary mixture will lean out slightly. THIS is what I believe is happening with your car.

The secondary metering rods need to go one or two steps richer to keep the mixture right after the secondary accelerator wells run out. These ports are located in the carburetor wall just above the leading edge of the secondary air valve.

You have a carburetor that we already know is jetted lean. The problems you describe are classic lean mixture problems. If you know the systems within the carburetor it really isn't hard to figure out.

If I were you I would tune the carburetor some before taking any advance out of the distributer. Your engine should run very well with 14 initial and 36 total.

BigBlockk

Later.....

 

BigBlockk thanks, i am understanding what you are saying....so if you were going to buy a second carb, one that would run good out of the box and allow me to disassemble this QJ and experiment with it, what would you recommend?

2101 intake with spreadbore. I want to keep my stock Cowl air cleaner.

 

A Jet Performance stage II #561-35002.

http://www.jegs.com/webapp/wcs/store...tegoryId=10271

BigBlockk

Later.....

 

So I set the initial timing back to 12*, and reconnected the Vac Advance to the program port. This caused a re-adjust of idle speed and mix. I talked the wife into going for a cruise after dinner. The car ran like a top and no breakup up to 5200rpm. Good response and smooth idle. She even commented on how well it was running.

I will conclude at this point based on these results that I have the max possible advance for the carb I have. ( at least for the way it's currently tuned)

I do think there is more power in this motor than I'm getting so I will continue too look at potential tweaks and mods. I'll check out that carb bigblockk.

Thanks for all the input guys.

ps; I love cruising under the stars on a cool evening. (or early morning works too)..put's a smile on my face ear to ear!!!

 

On the timing..when you say it has 36* total at 2500 rpm...are you saying that is with vacuum advance?

Let's leave vacuum out of the equation and get total to the 36* with no vacuum...then see what initial is. My gut tells me the dist curve isn't right and that when you bump initial up to the 14-16-20* range at idle (no vacuum) you are getting way too much total advance.

Initial= Idle..no vacuum
Total= reved up until it quits advancing..no vacuum

THEN you add vacuum to the deal and see what it will take for nice manners. Often it needs to be limited in dist or in vaccum can once you dial in some good initial and total curves. You can easily end up with 45-50*+ with total AND vacuum added together.

I think you need to do some dist recurving first.

If the bowls are truly running empty..then you can add all the jet you want...they are still going to be empty. Running out of fuel is a pump/delivery issue.

Don't disagree that you might be running out of fuel...but you need to get ignition squared away first. 95% of carburetor issues are really ignition issues!

JIM

 

i agree with 427 hot rod on the timing curve being the problem. first disconnect the vac advance and check the total advance you should have 30 plus degrees at 3000-3600 rpm if you do not that is most likely the problem. i would suggest finding a shop with a distributor test stand to curve it for you. if you can not find a local shop i believe Lars can do it if not let me know henry @ oles carb

 

There is another situation that might explain your problems.

If your spark plug's heat range is too hot, it can cause backfiring under full or heavy throttle load at high revs. If you can rev to redline under light throttle this could be your problem. Try plugs two ranges colder. I had this problem when running my L-46 with slant plug 2.02's with 11:1 CR.

It took me a LONG time to figure this one out.........

 

Guy's, you may be on to something with the curve thing. I'm gonna do some testing today and take readings with the VA in all three positions. Clearly with the VA connected to manifold and the inital set at 12*, it does not behave. Changing the VA to ported source, it all get's better. Vacuum reads 20in @850rpm on the manifold port. On the program port it reads 0in at idle, but quickly climbs with throttle and hit's 15in by 2000rpm. I'm wondering if it could have too much vacuum advance?

Stickshift...good thought on the plug spec. It i s 250hp 9:1 compresion motor. Running R44T. I won't introduce that variable just yet, but i'll keep in it mind if this persists.
Certainly be cheaper than buying a carb.

 

Hi Guy's,

I needed to resurrect this cause..well...I still have the problem. I have triple checked the timing and advance. The suspects are either points breaking down or carb as suggested by Big Blockk. The points show steady dwell when measuring rev wo load.

So, the short question is; can I feel confident that if I purchase the suggested carb:

http://www.jegs.com/webapp/wcs/store...tegoryId=10271

that it will perform out of the box and eliminate the carb factor? Thats a lot of money for a carb, and the current driveability of the car is excellent. I don't want to spend $360 and still have carb issues. But i really like the idea of fixing my problem and having a QJet on the bench to play with.

Also, while it was breaking down at higher RPM's it was blowing brownish looking smoke out both sides. Can I assume that is carbon burning due to lack of fuel?

thanks again for all the help.

 

doso -
I looked at your timing numbers in your early post, and your timing is fine. 14 initial, 36 total at 2500, and 30 at idle with vac connected. That's perfect. Leave it right there.

The EDL Q-Jet carb is set up very lean in stock format. I have tuned a lot of these carbs on my Tuning for Beer Tours, and your problem is common. The jetting in that carb is the Chevy Truck jetting, and it's extremely lean on the primary side at WOT, and this will cause the car to "lay down" and backfire at WOT. Try a jet/rod combo on the primary side of 74/43 and make sure you don't have a fuel filter installed anywhere on the suction side of your fuel pump (between the pump and the tank). Then, set your float level right at 3/8" (the stock level is lower than this). I'll bet this will fix your problem...

 

he already has a better carb! That Jet is just a reman, the Edelbrock is a new carb.

My thoughts (after reading these other posts) would be to check fuel pressure and volume...especially under load or at WOT. Lars did my Edelbrock on a beer tour - yes he knows them just like he knows the GM ones.

 

He wanted a carburetor to keep the car on the road while he tinkers with the original. Several members have the Jet Performance Quadrajets and none have complained about them. While I don't own one myself, they appear to be quality product.

The Edelbrock 'Jets were advertised as 50 state legal (lean jetting). The Jet Performance 'Jets look to be better suited for our cars.

Just trying to help.

BigBlockk

Later.....

 

Thats what I want to do...I think. There are many events I want to attend this summer, and my time at the bench is constrained by life's responsibilities. I have rebuilt a few Holley's in my past, but never have I had a Q-Jet disassembled. I think the EBrock could use a complete rebuild as it also leaks down over night. So I would like to keep the car running, and then take my time rebuilding and tuning the ebrock carb while learning the ins and out's of a Qjet. I assume the Ebrock version is reflective of an original in most area's.

I just wanted to get some confidence that the Jeg's carb will cover me while I take my time tinkering with the E-Brock.

Lars, If you think I am being silly and wasting my money please tell me.

One last question, where can I buy a re-build kit, jet's etc..for the Ebrock 1901?

thanks everybody for your input. It is very much appreciated.

 

If the man says this will fix it, then, this will fix it...jets and rods gotta be a whole lot cheaper than a new carb.

All bow down and worship the guru Lars...

 

Rebuild kit #350-1920 will do the 1901 carburetor.

http://www.jegs.com/webapp/wcs/store...tegoryId=10275

The Jet Performance Streetmaster Quadrajet is not just a reman carburetor. It has been modified and tuned to a certain extant. Now Jegs has a reman carburetor (555-15800) that is simply a rebuild. I would not get one of these UNLESS you are prepared to tune it.

With a strait reman being a "1", as far as being tuned to your engine, and a full boat Lars tuned carburetor being a "10" the Jet Performance Streetmaster is about a "7". Maybe a little more. Your 1901 is pretty close to the reman, a "2" or "3" maybe. You have to remember, the 1901 was a "fits all slap it on there at 8PM to get you back and forth to work in the morning while making Edelbrock a pile of cash" carburetor. Why do you think they make a tuning kit for it? No, I am not dissing Edelbrock. I just wish they had tuned the 1900 series carburetors for the other 49 states.

Bottom line. If this were my car, I would get the Jet Performance carburetor so I could enjoy my car this summer. Then I would get the 1901 rebuild kit (350-1920) and the tuning kit (350-1991) so that this fall I could mess around with the 1901.

http://www.jegs.com/webapp/wcs/store...tegoryId=10285

It is very rewarding when you finally get it running right, and you did it yourself.

My little half assed rant was not really aimed at anyone. I'm simply trying to say that it will cost some but it will all be worth it when you can stand back and smile to yourself when it runs good.

BigBlockk

Later.....