Backfire at high revs

[mattsay]

Hi People,

Apologies for not posting for a while - long story!

However, I have developed a Backfire (exhaust) at high revs and wondered if the good people of Stagland would help narrow down the search for the problem.

I run elec ignition, Weber, have good plugs (I think), pretty new dizzy cap, HT leads, coil and timing bang on (by strobe).

Any thoughts would be very welcome.

Cheers,

Matt.

[martin]

Hiya Matt, that sounds like an exhaust leak, check all the joints carefully, including the down pipes and the balance pipe. It might be a valve issue but not very likely. Martin.

[KOY 23]

You may be running too lean. My Weber was, although not backfiring. Vmad will know if you have the right jets.

[flying farmer]

Running lean is a favorite, the mixture is too weak to ignite by spark, but will ignite in the hot exhaust.
Another possibility is inadequate coil. I stupidly managed to snap a terminal off my lumenition coil recently on the stag engined TR, so thought I would fit the better quality coil supplied by LD parts, however I wasted my money as it was misfiring over 5500 rpm at full throttle, and a new lumenition coil fixed the problem.
It is significant, however that my engine is producing over 200bhp at 5500rpm, so the spark has to jump through a significantly denser mixture than with a standard engine. I once tried a standard coil and it wouldn't rev past 4000rpm!
I would check the jets first, if they are ok, suspect the coil
Neil

[mattsay]

Hi Guys and Dolls,
Thanks for the input. Chris (VMAD) has suggested different jets. They are ordered and will arrive tomorrow (Royal Mail 1st class running like clockwork as we all know, especially Saturday's when we will all need our saviour parts!!!). I will try the jets first as a low cost solution - one day hopefully!! Coil is new sports coil paired to elec ignition so hopefully that wont be the problem.
I do suspect my distributor however (especially the vacuum advance) but I don't know if this has ever known to have caused backfiring???
I am down on power after about 4,500 revs. Until then it's fine even under load. Hope it's the jets but this is a new problem so I am not confident it's the Weber.
I'll also check the exhaust for leaks as per Martin's suggestion - never thought of that, maybe a blinding shout!
Cheers.

[KOY 23]

What size jets did you order? Check that they send the correct parts. Webcom took 3 attempts to sent 2 main jets.
John

[Ian Durrant]

The vacuum advance being faulty would just increase your fuel consumption. As you have electronic ignition I doubt the usual distributor suspects of shaft bearing wear or base plate wear would be the cause either. is the electronic ignition new? when they go bad it can cause the symptoms you re getting. It could also be plugs (are they clean,the right temperature range and gapped correctly?).

Basically anything that can cause unburnt fuel to be in the exhaust at high revs will cause a back fire. Too rich can do it too, (dirt in the fuel causing the needle valve to flood the float chamber).

Check your mechanical advance is working using a strobe too, if its isn't then at high revs the burn in the cylinder will start too late and the burn will be incomplete when the exhaust valve opens, dumping fuel into a hot exhaust.

can't think of anything else that others haven't already mentioned, but as always with troubleshooting, check everything methodically to hve any chance of tracing the fault.

Hope that helps
Kind regards
Ian

[Andrew S]

Hi Matt, you say high revs, what range are you talking about 4-5k or 5k +?

[mattsay]

Hi John (KOY 23)
Ordered 140 main jets (up from the 135 on the original kits) and 170 air correction (down from 180 on the original kits). I ordered from Fastroad not Webcon, as I preferred their website.
Cheers,
Matt.

[mattsay]

Hi Andrew S,
I'd say 4500 plus.

[Andrew S]

Sounds like fuel starvation, but as Ian commented, check the basics first just in case

[StagnJag]

Did you time your ignition from the timing quadrant/pulley? From what I understand, they are not necessarily accurately positioned.

Just a thought.

Dave

[mattsay]

Thanks for the comments Ian. The plugs are gapped to 25 thou (newish plugs as recommended from the mighty Forum) and after cleaning and after a good thrash the electrodes are a nice brown colour (i.e. not rich and not white - lean) but maybe a fraction leaner on 4 of the pots than the others but you have to look hard. The mechanical advance is working fine - on strobe with revs from idle the timing marks advance to the end of the scale no problems. I am hoping it's the jets. we will see.
Cheers, Matt.

[KOY 23]

Same as Webcom suggested to me. My AFR is now spot on when cruising. Sounds like yours is running dangerously weak. I would be interested in knowing the results of your jet change. If it sorts it then I hope Vmad will contact all customers to suggest revised jets before an engine is damaged.
John

[mattsay]

Hi Dave, yes from the pulley and I agree that's no gaurantee of accuracy. It's because this is a new fault that I'm concerned. I would certainly bet that the fault has already been identified by one of you guys though. Let's see who's right.
Cheers and thanks again for all the advise.
Matt.