Race Engine Forum UK

Hello,
I've tried to solve this problems in many ways, but it's going from one side to another, and I can't find the optimum.
My engine configuration:
1,6 SOHC racing engine, CR 12,2;1, camshaft 316/304, 4-1 manifold, 2 x Weber 45DCOE 152, programmable 3D ignition system.

Weber setup:
38mm venturi
155 main jet
170 air correctors
45 pump jets

and now the problem: i've set up the carbs as above, plus 55f8 idle jets. The idle was fine, buti have to open the throttle on idle speed screw quite much to get the engine running, there wasn't back-fires in the intake manifold if I screwed in the idle mixture screws. But there was other problem - the engine was going down from the high revs to idle speed very slowly, and when I was accelerating and engage clutch to change the gear the engine was revving high by itself and still it has sometimes back-fires.
Then I've checked the typical jetting for similar engine configuration, and I found out that the idle jet is too big - I've changed it to 45f9. The back-fires disappeared, but the engine was very weak on the small load and the revving problem still occurred.
I thought that the problem should be in this throttle plates, which were opened too much on the idle speed screw. I found the good tutorial how to set up idle system. When I've checked my configuration and compared to the configuration from the manual (max 1/2 turn on the idle speed screw and max 1 1/2 turn on the idle mixture screw) I found out that to achieve this values I need bigger idle jet - I've put the 65f8 and do what was written in the tutorial, the idle now is quite good, the engine response is much much better, but it's getting back-fires which damages the manifold gasket. I have no idea what to do now. Now the engine is running fine, but this backfires kills the gasket - it can't be like this.

Below is my ignition map.
Please help
Konrad

Firstly do a compression test.

GC

12,5 on every cylinder

We'll have to go one step at a time. Compression shows that valve are sealing OK.

The cause of the engine over-revving when you want it to slow down could be a sticking throttle plate. You better get the carbs off and inspect this.

Incorrect ignition advance could be the problem too or faulty distributor.

You could also have excessive back-pressure from the silencer.

Please investigate all these things carefully,

G

1) Ummm, what is the camshaft timing?
2) Load sensor TPS?
3) When does backfires occur, full load or light load?

As I do work with such engines on daily basis, I'd say, that ignition on full load is sort of very, very conservative and quite much advanced for idle.

hi and thank you,
throttle plates are ok,
overrevving stopped.
now the problem is no power on the high revs - it has 123hp at 6344 rpm and should have over 130 around 7200rpm.

The setup now is 155/190, 38mm venturi and the ignition like below. 110hp was on the 0 degrees timing, then we've retarded camshaft about 3-4 degrees, but I think we have to retard it one tooth to set around 7-8 degrees retardation.
sorry for poor quality of the graphs, but my scanner is nearly dead.
best regards
konrad

P.S. load sensor is TPS

oh, and the backfires occurs on the low load, mainly when releasing the throttle

I have run thru some of the mechanical things that can cause backfire but the persistent backfire bothers me. To backfire out of the inlet there must be burning mixture in the cylinder. The primary cause would be ignition retarded (still burning on valve overlap) though one might perhaps also consider a spark occuring in the wrong place which I suppose could happen with a dual coil setup.

There is a serious problem here.

GC

the backfires has stopped when I've changed the idle jet to the small one (45f9), but the engine was very lazy then
konrad

Well, all that does is lean out the mixture.

As a last resort I am going to suggest you have either high exhaust back-pressure or the layout of your header is wrong because the there is something wrong with the firing event.

I don't have any other ideas and backfiring in this way MUST have a cause and the effect is symptomatic of a serious problem somewhere.

GC

Hello.

We also have same problem on our Fiat 1300 sohc engine, compression test was 15bar on all four cylinder (because of high CR and wery short duration camshaft). It also backfiring thru the carb on high rev. We also changed air correctors with bigger ones that this problem was stopped, but engine was wery lazy, like you say konrad. When I open the engine and see the valve seats I see that the problem was there, one inlet valve seat was cracked, so valve seal ring was cca. 0,5 width on some places, on high rev. valve did not seal 100%, but on compression test did seal. We changed cylinder head for another big valve one, and car was work 300% better.


This is only my expirience, maybe you also have some bad seal valve.

best regards, Grega

ps: what are you want to do with retarding the camshaft-lover pressure (later closed inlet valve) ?

http://www.youtube.com/watch?v=lSctdru2ATE

here you can hear the engine...and something more ;) the backfires occurs on the low load and rather lower revs, they can be heard on the movie clearly

Sounds like carb syncing is off. At idle it is missing is a few cylinders . Typical of duals out of proper synchronization.
Just my take from what I heard.

carbs where sync with synchrometer and then i've checked the position of the throttle plates via progression holes, so the synchro should be ok. It's spitting from the inner 2 cylinders only...hmmm...now it's not because the engine is in several pieces ;)

Syncing at idle is one yes but also as the linkage begins to load/move the throttle plates they must do so evenly. Your video mimics what I have experienced. Seems like it only occurs when the throttle is opened a bit and not a dead idle?

Do your dual coils have dedicated secondary windings for each plug that are fired independently by the control box? Or are two plugs sharing a secondary?