57 replies to this topic

[EunUCh]

sounds "about" right on idle jets if not back firing on back off down hill.

The back fire on "take off" does sound lean ,although the mains are "just under" 202 specs. they could be a bit on the "high side" with F16 tube ?

causing the back fire initially but then running rich as with a bigger main jet.

This could be 2 things the way i see it ...mains too big causing stinky fumes combined with a "lean" tube ? 

Lean tube not pulling over earlier enough but when it does the main is too big?..stinky fumes/blue smoke..oil wash?

[jd lj]

You really don't want to wash the oil of the bores otherwise you'll glaze them and it will need to be rehoned. It's not a good idea to run in an engine on poorly tuned triples or any poorly tuned carb for that matter. Glazed bores can then lead to sump pressure due to the rings not bedding in and sealing, this will then cause oil leaks, probably from the rear main etc which means that the engine is coming back out to fix that one.

I'm sure that your 60f8 or whatever size they were are miles too big. If you have problems taking off this can also relate to the idle jets. You can actually drive the car with the whole emulsion tube, main jets and air correctors assembly removed but only up to 2.5 -3k rpm, the mains etc are doing nothing below these rpm's so they aren't the problem with taking off from the lights etc.

Put a mark on one of the idle mixture screws and the carb body then turn it clockwise until LIGHTLY seated counting the number of half turns required to do so. Then wind it back out to make the marks line up again using the same number of turns as you wound it in. I would expect them to be less than 3/4 of a turn out which will also indicate that the idle jets are too big.

[RallyRed]

Hi..
Im no weber expert...but had my car dyno'd in late december...was a eye opener to see how easy all this is..if you have the right equiptment...the guy just stuck the sensor up the exhaust pipe...and, as well as all sorts of other dyno stuff...he could see within a minute that it was leaning out at about 5000rpm. Thus upped a few jet sizes..redid test and graph was now showing correct mix. Maybe it was an easy one.. and he was good at it too...but it was all so obvious right there and then. Dyno time not cheap..but easy to do back to back tests very quickly and assess effect of the changes made. No new news there..I was just suprised how easy it seemed to be to nail an issue. Good luck too.

[LC-GTR-1969]

Ok so after much consideration and with the help of the people on this thread, it seems like the engine is running too lean! (With the back firing at initial throttle and now white spark plugs I think its safe to say it is running lean) Totally the opposite of what I originally thought.
 
So, if the engine is running lean could this be a cause of my flat spot at initial throttle.
 
And is there any internals of the webers I can change so that they don't run so lean.
 
Thanks

Have you checked for Intake vacuum leak?

I would be surprised if you were lean based on jetting, but vacuum leaks will cause lean condition...

[jd lj]

I'm thinking that to compensate for a slightly rich idle jet that the idle mixture screws are wound to far in and then to get it to idle the throttle plates are partly exposing the first progression hole to achieve an idle, this can also cause a stumble due to leanness on initial throttle.

When you come off the accelerator pedal will it drop back down to idle straight away or does it take a bit longer to do this?

Remove the progression hole covers and have a look at where the throttle plate is in relation to the holes, you'll be able to see the brass throttle plate when you shine a torch in there. The throttle plate should be positioned directly below the hole nearest the engine, you shouldn't be able to see either the front and definitely not the rear edge of the throttle plate through the hole.

There are many parts that you can change to adjust the air/fuel mixture, but first you need to determine what rpm range needs to be changed, this will then determine which jets need attention.

Ps. Where are you seriously located, eg Sydney, Melbourne, Brisbane etc (not your home address). Also who tried to tune them for you previously, possibly good if other members don't use them for tuning webers. Who knows they might be better with other carbs.

JD

[jd lj]

To check for vacuum leaks squirt some water around each end of the manifold and listen for a change in the idle.

[caterham2]

 

To check for vacuum leaks squirt some water around each end of the manifold and listen for a change in the idle.

 I have a number of Weber Factory publications and in their texts the way to test for vac leaks is with a small squirt bottle with................ get this................. PETROL IN IT. I am sure you get a very convincing change in engine sound doing this if you have a leak somewhere BUT over the top of a hot exhaust system on a 6 cylinder Holden will get some pyrotechnics as well I would imagine. On a more serious note to the O.P. if you are not able to pick rich from lean then almost certainly detonation and ruined pistons are coming your way. In a previous post to you I suggested that you take the car to someone who knows what they are on about, weeks later and much stumbling in the dark you are no further down the track to a fix, if you don't actually live in Bangladesh I may be able to point you to someone or 2 or 3 that will sort for you.

[jd lj]

Yeah, I've read about using petrol or wd40 to test for vacuum leaks but I'm not about to recommend it to anyone, water is a much safer option.

[jd lj]

Let us know what the outcome is and what jetting specs it ends up with and how it drives once done. That way maybe we can all learn from it. I hope they sort it out for you.

Did you check the position of the throttle plate compared to the progression holes?

[caterham2]

 

Thanks again. caterham2 I have taken it to a weber specialist, and I am taking it back to him. I am just trying to learn as much as I can from this forum so I kinda know whats going on.

 Not the same "specialist" that "set the carbs up" for you last time I hope.

[orangeLJ]

Sending carbs off or having someone work on them off the car is fruitless.

Dyno time or road tuning with an exhaust bung is about the only way you'll get them perfect.

[warrenm]

What orangeLJ said.

[jd lj]

I think he's sending the whole car not the carbs by themselves, as you said that would be pointless.

[Bigfella237]

83 kw = 111 rwhp = 27.46 cc/hp

 

I would have expected it to do a little better than that?

[jd lj]

I'm glad you're happy with the outcome. Now it's time to go and enjoy it.