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Discussion Starter · #1 ·
Just got my '56 Enfield 700 broken in and have been doing lots of riding around town, with 110*+ heat. Bike pings while accelerating from a stop sitting at a light, or when I accelerate to passing speeds.

Bike originally had NGK B7HS plugs with stock 7.25:1 comp.

I upped the compression to 8.5:1, has NGK B9HS (2 heat range colder) plugs and set the timing to the retard setting within the stock timing range (full advance 3/8" - 7/16" from TDC. It's currently set at 3/8" before TDC)

Should I get colder plugs and/or retard the timing more? Normal riding has brown plugs with no indication of detonation/overheating, but I hear it and know my limits when riding around to not have the bike ping. Trying to get this bike good for a 2 hour ride down to the Born Free show on the 30th!
 

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Discussion Starter · #3 ·
That worked with the low end when I screwed the idle mixture screws in, but thought it had more to do with timing/ignition than mixture.

I'll adjust the needles (currently on the leanest setting) on my lunch hour and see what happens. Thanks Tony.
 

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Excessive timing advance is the usual culprit. Try retarding the timing a few degrees at time, that should do it. And as Tony says, too lean is also no good.

If you're lugging the bike (low rpm + high load), that can make it detonate. (Point of peak pressure and crank angle, etc.)
Remember how Romero won Daytona (or was it Ontario?) on the Triumph triple when all the other factory bikes detonated themselves to pieces? He did it by keeping the rpm up.

It's normal to not want to rev up a freshly rebuilt engine, but too much babying is also no good.
Spark plugs won't do much for you--run 7s or 8s.
You can also try using a higher octane fuel, however, the ethanol in the gas these days actually helps to suppress detonation.

It's also possible that you're running a higher compression ratio than you think. If all else fails you can use a slightly thicker head gasket. One of the problems with high-comp pistons in old Brit bikes is that they make an already inefficient combustion chamber shape even worse. Not so bad back in the day when even pump gas had high octane ratings, but not so good these days.
 

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Hmmm, I agree with Torch. I'd suspect that your mixture is too lean. Typical culprits are too lean of a mixture, too advanced timing, and carbon build-up in the cylinders. Given the number of miles on the engine (new?) and the fact that you appear to have carefully set the timing...I'd bet on mixture.

Edit: You may also consider backing the timing off just a hair since you raised the compression considerably.
 

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3/8" BTDC is likely to be less than 35 degrees advance,which doesn't sound excessive.With a conrod and stroke length,you could figure it out exactly.
If you get "pinking" at 1/2 throttle,raise the needle to richen it.

Do jet tests at full throttle and high rpm.2 sizes smaller than the smallest jet that causes missing and 8-stroking,is usually about the right size main jet.It takes about 3 second to know if it's 8-stroking.

8.5:1 is fairly high for a big single,but you should still get away with B7 or B8 plugs.
 

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Discussion Starter · #9 ·
Thanks for the input thus far, guys! Yup, it's a twin and has about 400 miles on a fresh rebuild with the new hi comp pistons. Most of the miles were done when it was still cold out and I babied the shit out the bike up till about 100 miles ago(when I started riding again), so I didn't notice any pinging then.

The off-line ping is gone and after lowering the needle 1 notched helped the pinging at 1/2 throttle, but still there after a long ride with lots of stop lights. It's 110*+ outside. Gonna try lowering the needle, then try retarding the timing a skosh if I still have a problem with it.

Should I stick with the coldest spark plugs? Currently a set of Champion equivalents to B9HS plugs.

Glider, took a ride with some buddies up a local mountain the other day, and had to stick in 3rd gear with the RPMs up to keep it from pinging. Sure would like to fix the problem and go back up. :D
 

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however, the ethanol in the gas these days actually helps to suppress detonation.
This is only true if you tune the engine to suit ethanol.It does have a high octane rating,but a 10% ethanol mix requires 4% bigger jets or you run lean.Ethanol also requires less spark advance.Some high octane fuels require even less advance,up to 10 degrees.
Without re-tuning the engine,it will knock more than ever at full load with ethanol blends.

I'd use B7ES plugs.Use B8ES if you really thrash it.
If it doesn't "pink" at high rpm,the problem could just be the advance curve.If the springs on the AAU aren't tight enough,it won't return to full retard at idle.It will be reaching full advance at lower rpm than it should be (too much advance at low speeds).Full advance at 3000 rpm usually works OK.
 

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You said you 'dropped the needle.' That's going to make it run leaner, not richer.

You didn't mention anything about going to a richer main jet--better try that next.
 

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Discussion Starter · #12 ·
You said you 'dropped the needle.' That's going to make it run leaner, not richer.

You didn't mention anything about going to a richer main jet--better try that next.
Ah yeah, I dropped the needle clip, not the needle. It is set to the richest setting - needle wise and I'll give it a run after work.
 

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Discussion Starter · #13 ·
After setting the needle to its highest point, the pinging stopped!
Thanks for the help guys.
 

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Discussion Starter · #15 ·
Tony is indeed the man!

Brief update: I said before the pinking stopped after the carb adjustments, but after the bike is warmed up and riding, I loose lots of power, as if the bike overheats. Cold running, the bike is a damn rocket ship then all my torque is gone after she's hot and hitting traffic.

I backed off the timing by retarding it to 5/16" (I wish I could use a timing gun for this shit) and will know how she runs after I get my new charging system installed. Is there a guide for piston length BTDC vs. timing degrees? Should I have adjusted the timing to different setting?
 

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I'd try advancing the timing just a smidgeon. Now the pinging is gone, the mix is possibly a little rich so bump the timing up and see if you gain some power when warm, Obviously listen for the pinging. Sounds like you are getting it dialed in.
 

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I backed off the timing by retarding it to 5/16" (I wish I could use a timing gun for this shit) and will know how she runs after I get my new charging system installed. Is there a guide for piston length BTDC vs. timing degrees? Should I have adjusted the timing to different setting?

you can use a timing gun, we strobe time all magneto bikes, here is how I do it.
all you need to do is make up an adapter to attach a degree wheel to the drive side crank sprocket nut or bolt and put a degree wheel on it, you need to establish TDC with a piston stop tool or a good guess with a screwdriver in the spark plug hole then put a wire pointer at the TDC position.
Then we lock the magneto advance in the full advance position and remove the spark plugs a using a spring clamp to clamp them to the outside of the head on the fins. then just spin the engine over (it doesn't need to run) and watch on the dregree wheel where it's firing at. We put a yellow mark at 38 BTC and it makes it a lot easier to see when it's spinning.
 

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Discussion Starter · #18 ·
I probably wouldn't complain so much if a simple timing change wasn't such a time consuming, messy ordeal. I've even thought of changing to a manual adjusting magneto and scrapping the auto advance unit. It's unfortunate those are pretty pricey these days.

I talk to an Aussie Enfield guru who has pinking problems with a bone-stock Interceptor, going up a hill on a hot day. The early Inters used a mag while the later bikes used a coil ignition. He suggestion I make that switch to a Pazon.

EDIT
Tony, thanks for the help, again!
 
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