82 replies to this topic

[liirge]

I think you could try having warmer sparking plugs, they could be wetting at low revs a warmer plug would help stop this. I had a similar problem with an engine that would keep missing on 1 and 4, increasing the spark plugs temp seemed to cure the problem.

So if you're on NGK plugs

Normal Mini ones are BP6ES
Hotter are BP5ES
Colder BP7ES

There are some other really wild things running through my head but I don't want to mention them yet

[Yoda]

Just throw them into the mixing pot and we can have a laugh see what we think

[liirge]

Absolutely not!
Not until warmer plugs have been tried, a lot of people go out and buy colder plugs when they raise the compression ratio, but this thing only has a 10.2 to 1 C/R and cold plugs are not necessary at this level.

[Yoda]

But why would one plug be dry and the other wet but when you pull their respective leads, you get no change in engine note? i wont be able to sleep now until i get an answer changing the plugs may correct the symptom but it does not solve the puzzle methinks. I still tend to think there is more than one problem. The dry plug could be a leak at the inlet not drawing enough fuel maybe, and the wet plug could be simply a plug or lead breaking down at idle. Both symptoms may improve at higher revs!

I ran 5's in yoda for a while in the 1380, but soon went back to 6's, but that issue was running too lean at high revs under load, a bit of needle profiling on the twins sorted that out. Handy things, nail files

Edited by Yoda, 23 August 2011 - 03:45 PM.

[Cooperman]

A set of the old Champion N9Y or even N5 would be worth trying if they are available.
It's hard to identify why, when 2 cylinders are sharing the same main inlet port which bifurcates just before the throat, the two cylinders so fed don't perform the same.
If the compressions are the same then it almost must be an ignition fault really, so it's back to dizzy cap, plug leads or plugs.
Round & round we go! Still, it's a good challenge.

[bigmatt4]

ok i have now fitted a stock standard closed chamber 850 head no porting no oversize valves good nick. started engine now hs a slight cammy idle but still cylinders 1 and 4 will not run properly. Better than the 1275 head.
removing the plug leads now make a little bit of difference . both dizzys that i have used have different leads all brand new (im an auto sparky) so i have come to yet another idea .
this engine as a 6" manifold on it looks to be more 4 a 45 not a 40 and it has had some serious porting so because i now have the 850 head on it and the ports are very small (only using head for diagnosis) . thinking if i put a 4" manifold on it with no major porting it will increase airspeed.
because im also thinking if you measure from the butterfly to the inlet valve almost 9" almost a full cylinder of air is sitting in the manifold and head which wen the valve on no2 closes the air stops moving and goes dead as such fuel falls to the floor and walls loses its atomisation.
looking at saimese port design wen you move the firing order back 1 step 1-3 4-2 to 2-1 3-4 you look at port no 1 cylinder 2 draws fuel first then immediatly after cylinder 1 draws. hence the dead spot in between same for 3 and 4. im definatly clutching at straws so ive worked out its not the head its not the compression or gaskets its not the cam or cam timing and its not the dizzy so leavs carb design and manifold length i have a 4inch manifold on its way so fingers crossed. bit afrad to run engine without pushrods because of the lifters slapping the cam unloaded and its not a scatter cam. I did notice running on the 850 head that the plugs stay dry
(found my wet plug problem to be oil down the guides wen filling engine with oil and forgot 1 valve guide seal)
but i have done no mileage on it to see as engine has no radiator or waterpump on it at the mo as i have been experementing with cam timing trying factory settings 103 deg and 106 intake centerline with no noticable difference. sorry my punctuation aint the best but if you ever spoke to me you would notice i dont use fullstops in my talking either aparantly i talk to fast:)

Edited by bigmatt4, 24 August 2011 - 09:34 AM.

[Cooperman]

It's hard to see how it could be the manifolding due to the siamesing of the ports. I take yopur point about inlet charge velocity, but that would normally effect the cylinders equally as incorrect manifolding has always done. Slow inlet charge into cly no.1 would also equal slow inlet charge into cyl no.2.
Maybe there is some strange swirling effect at the point of inlet on the two end cylinders, but it must be very rare as no-one on here seems to have ever encountered it before.
You have certainly got a lot of us scratching our heads.
We'll all keepon thinking about this for you. I'm sure we'll get there in the end.

[liirge]

Sorry I need someone to summarise that response, i Honestly can't make head nor tale of it.

Am I right in saying that it is firing on four now, with an 850cc head on it.
But 1 and 4 only make a very small difference to the engine note when removed?
And the OP thinks that this has something to do with inlet manifold tuning?

Have I got this about right, as that's all I could really translate.

Edited by liirge, 24 August 2011 - 11:36 AM.

[jaydee]

From what i can understand..he said that the manifold is too long, causing poor fuel atomization, so petrol is pouring down the cylinder and flooding it.

Matt, well it could be, but please can you post a close pic of dizzy, leads and coil connection?
In the one you posted before it seems like you fitted the leads the wrong way round.
Also, please put cyl 1 on TDC firing, remove the dizzy and take a pic of the dizzy camshaft in the engine too..

[Yoda]

Thats how i am reading it ( i think ) but i am still not convinced with the new theory. As Cooperman says, it makes no sense with the siamese port layout, but hey, stranger things have happened! Obviously the engine has been built as a bit of a Heinz 57 (thats a "Bitza! to the younger generation "Bitza this and bitza that" ) So a few teething problems are usually inevitable. but this really has my head spinning.

I agree with the though of running the engine with no rods and leaving the followers in but for a short duration it might help prove a few points as discussed earlier. Its worth a try anyway because if we cant sort it out, it will end up as scrap value regardless.

The wet plug may well be caused by the reason you have come up with but have you by any chance tried changing the plugs at all? i mean a fresh set out of boxes, not just swapping them around. I have several times in the past, gone out to customers who state quite clearly that they have changed the plugs, only to find them rummaging in the used spares box for more! and a new set swiftly fitted by me has cured the problem. I am not undermining your experience but sometimes the obvious can beat the best of us.

I am still convinced that the problem has something to do with ignition rather than fuel.

Edited by Yoda, 24 August 2011 - 01:32 PM.

[Yoda]

From what i can understand..he said that the manifold is too long, causing poor fuel atomization, so petrol is pouring down the cylinder and flooding it.

Matt, well it could be, but please can you post a close pic of dizzy, leads and coil connection?
In the one you posted before it seems like you fitted the leads the wrong way round.
Also, please put cyl 1 on TDC firing, remove the dizzy and take a pic of the dizzy camshaft in the engine too..

I have already asked for this so hopefully it is forthcoming soon.

[jaydee]

From what i can understand..he said that the manifold is too long, causing poor fuel atomization, so petrol is pouring down the cylinder and flooding it.

Matt, well it could be, but please can you post a close pic of dizzy, leads and coil connection?
In the one you posted before it seems like you fitted the leads the wrong way round.
Also, please put cyl 1 on TDC firing, remove the dizzy and take a pic of the dizzy camshaft in the engine too..

I have already asked for this so hopefully it is forthcoming soon.

This should clarify us some things i hope!



Plus one on the spark plug idea, i'd suggest to fit some new champion N9Y plugs, although i've never had fault with ngk 6's, champion plugs are another league.

[Yoda]

Just a silly idea, but sometimes they are the best.

I dont suppose your firing order has been set clockwise by any chance? that might explain a few things. so a picture would be really useful here.

[jaydee]

Infact it seems that the leads are put in some clockwise order, but with something else wrong because if the leads were only clockwise, engine wasnt starting anyway..
This is an odd one lol

[cooperrodeo]

It might be the effect of the cam and overlap of the valves. I had what I thought was a strange misfire on cylinder 3 of my Spitfire 1500 using a Piper 270 cam with nothing wrong with the compression. Using a Gunson colourtune plug I could see random blue flame, yellow flame and no flame at all. It was better at a higher idle speed.

My 1275 has a slight tendency to misfire, also on cylinder 3 at low idle speed, but it goes perfectly well otherwise.