correct timing for a piper 285 cam in x-flow

[mazie]

hi all, still having problems getting the timing set correctly, have a piper 285 cam and electronic ignition, the manufacturer of the dizzy (bestek)gave me a value of 12 degrees at 1000rpm, now when set, low and mid rev's run fine but it starts to miss and wont rev past 5800 on an engine that used run to 7000 with ease, i tried to advance and retard the ignition all with a timing light , but i found it back fires through the dellorto carb at low revs or misses completely when full throttle is applied at low revs say 1500 in a high gear.

Even on the rollers we were struggling to get it all running smoothly all the way through the rev range, my knowledge and experience is now lacking somewhat, Do i have to send the dizzy back to get the advance curve altered or am i missing something?

Was planning a long drive in the car in a weeks time and im not happy going anywhere until its set.

any help much appreciated.

[Justin S]

back firing through the carbs is usually because it is running lean. Is it only the distributor that you have changed?

[mazie]

the carb had not been rejetted yet, still unsure of what base setting to go from, all i new was the single 45 dellorto used to on a 1380 mini racer, its now on a 1710 x-flow with a nice ported/polished head, balanced engine piper 285 cam, stainless manifold to side exit on a kit car.

[Rocket Pepper]

I'm assuming you have the cam timing correct. Backfire through the inlet, I'd be doing a leak down on that motor. It's possible you've got an inlet valve issue.

[mazie]

yes was set correctly, im only having this issue after changing from standard lucas points distributor to full electronic, before this the engine had run fine except i wanted more reliabilty so bought the new dizzy.

[Rocket Pepper]

I'd still check leak down, just because it backfires. So many times I've ignored preliminary because I knew, only to go back and find something that was fine had now failed even though unrelated to where I was at.

[Justin S]

put the old distributor back in and set it up.This will then show that that is the fault and start again. Back popping out the carb can be caused by the timing or from running lean.I don't think a leak down test is really necessary.

[mazie]

I really dont want to leak down, i will replace the dizzy and go from there, one last thing, is it the cam profile or the dizzy advance curve that dictates the way the timing is set?

[Mr2Mike]

mazie said:

I really dont want to leak down, i will replace the dizzy and go from there, one last thing, is it the cam profile or the dizzy advance curve that dictates the way the timing is set?

Both. The distributor advance curve needs to be tailored to the engine, the cam, compression ratio, exhaust, head etc. all affect how much advance is needed at any particular RPM/load. Unfortunately a distributor ordered with a preset advance curve is always going to be a best guess, the only way to get this half right is to have a distributor recurved on a rolling road.

That said even the most well set up distributor is a compromise, especially if you aren't using any vacuum advance (e.g. with side draught carbs fitted). A mapped ignition system (either commercial or DIY e.g. Megajolt) can be tailored to the exact requirements of the engine.

[That Daddy]

Mr2Mike said:

mazie said:

I really dont want to leak down, i will replace the dizzy and go from there, one last thing, is it the cam profile or the dizzy advance curve that dictates the way the timing is set?

Both. The distributor advance curve needs to be tailored to the engine, the cam, compression ratio, exhaust, head etc. all affect how much advance is needed at any particular RPM/load. Unfortunately a distributor ordered with a preset advance curve is always going to be a best guess, the only way to get this half right is to have a distributor recurved on a rolling road.

That said even the most well set up distributor is a compromise, especially if you aren't using any vacuum advance (e.g. with side draught carbs fitted). A mapped ignition system (either commercial or DIY e.g. Megajolt) can be tailored to the exact requirements of the engine.

I had a nightmare of a job doing this in the early ninties on a highly tuned X Flow,with bob weights it really is compromise and not a good one,the Emerald ECU had not been out that long at the time but what really swayed me at the time was to at least ignition control my motor,but i blew all my money on the motor(if i knew then what i know now)you really need to get at least modern ignition control on that X Flow otherwise you are missing out big style carbs still have life in em, but a conventional dist system with bob weights i ran 33 degrees at 4K RPM(motor powered to 7.6K)and the rest especially lower down was a massive compromise,i would love to play with that X Flow now,the dick that purchased the car dident even respect the spec of the motor

[Rocket Pepper]

Leak down takes 5 minutes. It's worth doing regardless is the point. At least it shows you've not bent a valve, broken a guide or burnt a seat out. Never mind the peace of mind it offers.

[mazie]

had a go last night and have reached some what of a compromise, smooth running from 1800 to 5900-6000, i will try a leak down, i have never heard of this expression before, could you explain what the procedure is?

had thought about megajolt, but the cost of this swings me to fitting something more modern, even though there is something about the old kent engine that i love!

[Justin S]

Leak down is not going to show a great deal other than a worn bore/ piston ring or a bent valve etc.If the engine was running smoothly before the change of distributor, then I don't think it would have a big loss of compression by changing a distributor. A leak down test is a form of compression check using an airline rather than turning the engine over and connecting a gauge to the plug hole.

[Rocket Pepper]

The O/P has run the engine at high RPM on a dyno with unknown ignition timing. The O/P reports continual backfire through the inlet at certain RPM. Given the problems described, there is a possibility of fluctuating manifol pressure/vaccum. A concientous mechanic scratching their head over a time consuming problem should consider the possibility of valve damage - hence a leak down test.

Given this is the Internet and I was trying to be helpful without misleading the O/P, why would anyone want to suggest ignoring mechanical integrity under such circumstances? It's not like I'm telling the O/P to remove the cylinder head or something. It's a five minute procedure fer chrisakes!

Over to you. I'm done.

[That Daddy]

Rocket Pepper said:

The O/P has run the engine at high RPM on a dyno with unknown ignition timing. The O/P reports continual backfire through the inlet at certain RPM. Given the problems described, there is a possibility of fluctuating manifol pressure/vaccum. A concientous mechanic scratching their head over a time consuming problem should consider the possibility of valve damage - hence a leak down test.

Given this is the Internet and I was trying to be helpful without misleading the O/P, why would anyone want to suggest ignoring mechanical integrity under such circumstances? It's not like I'm telling the O/P to remove the cylinder head or something. It's a five minute procedure fer chrisakes!

Over to you. I'm done.

Good advice,its a tuned X Flow if its been run over advanced or weak then any kind of damage may of occured to either valves or pistons/rings,check the basics 1st then get back to the techi bits,i have seen many people get clobbered on stuff like this(yes even fairly tech savvi)