Strombergs Running Lean

[SherpaPilot]

I have a 72 TR6 that I owned for 20 years. I have had the usual problems including leaking fuel caused by bad float valves several times. About 3 months ago the forward carb began leaking. I took both off, replaced with grouse jets, new floats, new diaphrams, set the floats for 17mm, with new gaskets in, I reinstalled. After startup and running for awhile, I found a loud popping noise from my exhaust. To make a long story short, I posted my problem on this forum and received several suggestions. One idea was to replace the throttle bypass valves. Although the carbs passed the bypass tests, I ordered up a kit and installed the new gaskets. Problem continued. I re-checked the float levels a dozen times, reset the timing, enriched the setting of the needles to the max. I checked fuel pressure from my electric pump (3psi). I replaced the fuel filter and several fuel lines. I checked the vents in each carb. After the 7th or 8th re-install of both carbs, its still running very lean. I put in brand new plugs (champs N9Y). The old plugs were definitely running too lean. I'm about ready to euthenize these carbs and buy new webers. Is there something obvious I have missed? By the way, yes I enriched by adjusting clockwise. I guess I need tow it to an expensive garage.

 

[poolboy]

Are you sure that the needles were still threaded onto the adjustment screw ?

 

[Got_All_4]

Throttle shafts worn and leaking? I just test drove a TR7 yesterday and the idle was very high and not able to adjust. The car seemed to have a lack of power too. I sprayed some brake clean at the rear throttle shaft and instantly the rpms came down. easy test.

 

[TR3driver]

Have you looked for vacuum leaks in other places, like the brake booster?

Checked that the carbon canister is not plugged? Emissions hoses routed properly?

For a lot less than Webers (which bring their own problems), you can send your carbs out to be professionally rebuilt. But given how much you've done to them, it seems more likely to be something outside of the carbs that is the problem.

Like maybe a cracked vacuum line. Or (now) missing restrictors.

 

[DougF]

Pull the fuel lines from the carbs and run the fuel pump for a second to confirm there is adequate fuel getting to the carbs. If not, check your rubber hoses for inner swelling.
Just spoke to an engine builder yesterday who has a customer that has cooked three race engines. Everything tested well, but the problem ended up being a piece of heater hose that was used instead of fuel hose inside the fuel cell. A lot of damage for a $2 piece of rubber.
The person who sold him the chassis didn't do him any favors.

 

[AltaKnight]

Assuming it ran correctly prior to your "fixing" the leak and installing new components then I would look to the fuel level in the carb float bowls being too low causing lean condition.

 

[SkinnedKnuckles]

Where were the floats before adjusting to 17mm? 15mm or 16mm? If you raised the height the fuel level will be lower in the chamber. That will lean the mixture for a given needle setting. Once the needle is out of adjustment range and all other options have been tried, lowering the float adjustment usually works. No more than 2mm at a time.

 

[SherpaPilot]

Hey guys, Thanks for the help. I hope to get back to this problem first thing on Monday. It looks like I need to get back into the floats and re-adjust. I set 17mm because that is what the book recommended but it makes sense this setting my be too tight since they were running correctly prior to the new floats as Brent and Graham suggested.

I never measured the original float levels during removal. What is your suggestion for a new initial setting? How much should I correct from 17mm?

Don

 

[AltaKnight]

Try it at 19mm

 

[SkinnedKnuckles]

Try it at 19mm

I'm pretty sure that's in the wrong direction. The measurement is for the needle valve in "closed" position & upside down, so increasing the distance will lower the operating fuel level. Try 15mm instead.

 

[DrEntropy]

 

[poolboy]

I have to ask; when do you hear this popping ?
Is it under medium to hard acceleration or during engine braking such as during a down shift to slow you down ?
Can you make it pop in anyway when the there is no load on the engine (ie in neutral) ?

 

[SherpaPilot]

Hi Brent,

I get the popping exhaust in both unloaded (neutral) as well as under a load. While under a load, letting off the throttle will produce the sound and aftet ignition cutoff, it will sometimes backfire through the carbs. I am very confident this is not ignition related. Prior to the dissembly of the strombergs, the only problem I had was the massive fuel leak caused by the failure of the float valve. Both carbs have indicated a leaning condition as indicated by all six plugs. The advice you guys have given me about the float adjustment makes sense. From what I understand, I need to adjust the floats to about 15mm which will increase fuel volume in the bowl. Is this correct? I have removed and installed these carbs so many times, I can do it in my sleep. Thanks for your help.

Don

 

[SkinnedKnuckles]

Hey, Don.

Yep. When I went to triples I got really good at pulling the carbs... Glad that's long over.

Try the 15mm - it should rich things up. Of course, there's needle mixture adjustments afterwards, so be prepared.

 

[SherpaPilot]

Nothing has resolved the lean running problem. The Strombergs were inspected by a professional British shop and given a thumbs up report. I changed out all vacuum lines and installed new gaskets. The engine runs like crap and overheats. I'm thinking I may have something internal or a cracked cylinder head. In any case, after three months of working the problem, I'm throwing in the towel and trailering the car to shop. The bill will most likely be more than the car is worth.

 

[AltaKnight]

leaking intake manifold/gasket?

 

[RonMacPherson]

I just scanned this thread, so got in recently..

I am curious as to the state of the engine.. Has the valve timing been changed? Have you done a compression test? If so, what was it? What is your vacuum gauge reading, at idle, under engine acceleration and running down the road...

Poolboy, is the guru who has some very informative links on vacuum gauge diagnostics.. And to me, after the compression testing I would be heading in that direction...

I recommend further diagnosis before tearing it apart. Either by you or a competent mechanic at a shop...

 

[poolboy]

Thanks Ron. I always keep these links handy.
https://www.secondchancegarage.com/public/186.cfm
https://www.centuryperformance.com/tuning-with-a-vacuum-gauge-spg-148.html
https://automotivemileposts.com/garage/v2n8.html
I was wondering if the needles were replaced, what with the out of spec needles sometimes found. That, and if the air valves were put back in the proper orientation, but he said that he had the carbs checked over,,so I kinda blew that off.

 

[ArcticOne]

Please keep us updated on this.

My problem is the exact opposite (I can't lean the carbs out enough).

It will be interesting to see what your issue is,

Although I am running twin Stromberg CD 150 on the GT6.

jb