400 Backfire

[Bill Campbell]

I finally put my 76 400 back together but she is not running as well as she did. I set the deck height at 1.5mm and timed it just outside 2.5 BTDC with an original Bing running a 200 main jet which I know is rich by spec. The deck height is different to the previous setting which gave me some pinging in the mid range. i now have an engine that starts easily but will not rev out in the mid range and even backfires.I have tried running with no muffler packing and no airfilter but it is still the same.Any suggestions would be greatly appreciated.

Bill

[joe novak]

Is the backfire a regular or irregular occurence?  When in the powerband does it backfire?  Low, midrange, top end?  Backfire may occur from a unreliable spark.  Check your ground contacts.  Be sure they are secure and clean of paint and corrosion.  joe

[DKWRACER]

Try re-checking the timing, woodruff key might be partially sheared...

Adios,
Tom Brosius

[SouthRider]

YES - Recheck the flywheel.

If it is an enduro model with the large motoplat - they were prone to shearing the mag key even from something as simple as a backfire or sudden stall (like from hitting a log & killing the motor). They had a small shaft, large flywheel, and a lot of inertia from the large piston & crank wheels.

If it has the large flywheel - lap it in, replace the key, and install the flywheel with a little blue loctite on the shaft (not the flywheel nut or threads). On some bikes we even had to use red loctite. Make sure you check the flywheel nut torque 2 or 3 times after getting the bike hot. Of course get the timing where you want it before you use the loctite.

These are high torque - low RPM short stroke motors - they didn't respond too well to advancing the timing much, and actually run better without bumping the compression.

Did you re-check the deck height AFTER torquing the head down & running it? New gaskets will compress a bit after use, and change the height. We used to mic the OEM gaskets & try to replicate the thickness as a starting point, but I guess that won't work anymore. It was actually quite a bit of work to get one decked properly.

I quess what I'm saying is that for the vast majority of riders these bikes were a lot easier to ride with low compression & mild engine timing. Otherwise they could be a beast that actually wore you out, made you slower, and sheared keys easily.

Two last thoughts - do you still have the screen on the bottom of the main jet in the float bowl? Without it you can get some surging that will have you going mad trying to tune the bike.

And lastly - make sure you didn't leave a rag in the exhaust port that is now throttling down the exhaust. That too has driven many a tuner crazy......

[rob w]

Bill, Sounds like you may want to check the tightness of your cylinder head. On a 400, you may need to re torque several times before it seats properly. I've experienced my 400 back-firing when the head was'nt tight.
Bob

[tomale]

Wow, I never had any trouble with my 400 with the exception of a shift shaft that need to be replaced but other than that it alwyas ran great..

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[Bill Campbell]

Thanks for all the replies. As I said I never had any troubles before except slight detonation in the midrange which I reckon was due to a small X distance. I pulled the motor apart because there was a strange noise in there that warranted inspection. Did not find anything wrong in the motor , thinking the timing may have slipped etc so I put it back together. I will check all of the above and see how we go.
Cheers Bill

[brian kirby]

If it has a Motoplat I would inspect the stator very carefully and make sure there are no cracks in the epoxy. Cracks let moisture in and eventually cause it to fail, but it does so slowly. My Maico 440 was hard to start, but didnt backfire so I thought it was jetting. It got harder and harder to start, but I kept chasing jetting. Eventually it started to backfire, which it NEVER did before. I still thought it was jetting even though I was using what everybody else was using, but as a last ditch I put in a PVL and it fixed it. All along the Motoplat was slowly going bad, I think the cracks let moisture in which slowly raises the resistance in the stator coils to the point it eventually fails, but Dane could probably give more details. I also agree with those that say check the woodruff key, especially if you are taking the flywheel off the check the stator.

Brian

[Rain Man]

wont rev up and backfires in the process of. Sounds like the midrange is too lean. That big ole 400 piston likes its fuel when you crack that Bing open.

Raymond
 Down East Pentons

[brian kirby]

Too lean a needle jet or clip position will also cause mid rpm partial throttle (in other words, not under heavy load) pinging too like when you are in a corner but you are not completely off the gas kind of feathering the throttle. I would still inspect that stator, get a magnifying glass out and make sure the epoxy is not cracked.

Brian

[Bill Campbell]

Sorry guys if you misread me. It used to ping when it ran a higher compression before the teardown. Now the deck height is higher so there should be less compression. it also is running a 200 main. Plenty I would have thought.

Cheers Bill

[Bill Campbell]

Just thought I would let people know that I think i have found the problem - a very lose ignition coil wire. Must have been the vibration as it was increasing in the rev range that caused it to misfire then backfire. Checked the pipe head torque and carby as well. it now appears to be revving out ok. Thanks all for the input

[dirtbike]

Just one thing, a higher X-value (thicker squish band) will infact increase the risk of detonation. The X-value should be kept under 1.5 mm in order to reduce the risk of detonation.