Topic: OK with warmer plugs? D7EA, D8EA get fouled. Too hot cam for 836? -Sorted

[PeWe]

I have had problems with fouled plugs when tuning my carbs, Mikuni VM29.
Still fouling despite one step up from far too lean jetting where engine had severe start problems.
Jets: main 117.5  - pilot 20 - air screws 0.5 turn open

It runs better with NGK D7EA than D8EA, DR8ES-L. 
I hear when revving the engine that it is a little bit lean, small lean explosions in exhaust when closing the throttle after 4000-5000 rpm when engine is warm and possible to rev beyond 3000rpm.

The idle is possible at 1400-1600, but it will run cleaner at 1800rpm and idling for a few minutes without stop when open the throttle again. Still accumulated rich at idle. D7EA much better.
- These warmer plugs might be too warm when driving the bike on roads in higher speeds?

I think that the cam is too hot for street use where idle is important. 
Cam CX-7 seems to be too much for street use OR it must have bigger displacement..... ? 836 too small?

I have read that very hot cams will go rich on idle due to the reversed airflow thru carbs at low rpms.
(Ignition is almost spot on F, leaning to late at 1600. Full adv @2800rpm)

I'm thinking about replacing the cam with a DP295 when the engine will be out anyway. With that can I finalize carb jetting.
Can use it for next engine project +900cc to which I have old ported head, 900cc cyl.

A friend back in the days had similar problem with his Kawasaki Z1-1000-77.
It had ported head, RC pistons and cams, Dellorto carbs that was popular here 1982, before the Vance &Hines parts arrived.
Fouled plugs at red lights and engine stopped . One day he had only one step warmer plugs that he used to be able to drive back home and found that this was the solution.

[Don R]

Much of my fouling/ running rich experience was caused by float levels. After I made a clear tube float bowl I was able to correct my jetting and now it runs awesome. Your mileage may vary.

[turboguzzi]

could easily be a weak ignition....

[Jerry Rxman Griffin aka MuthaF'er]

Hot coils??

[PeWe]

Much of my fouling/ running rich experience was caused by float levels. After I made a clear tube float bowl I was able to correct my jetting and now it runs awesome. Your mileage may vary.

I have not done the practical test with float levels by clear tube. Only a quick check when I had the carbs on the bench. All were the same and at recommended level. It would be nice with transparent float bowls and always see the status. Haven't found for VM29 (yet)
-The engine runs much better now with one step warmer plugs. Flowing floats should be a problem with warmer plugs too, or...?

could easily be a weak ignition....

Ignition should be OK. Pamco Ultimate kit with coils, wires, NGK caps. The caps are 5k caps and might not cooperate that well with resistor plugs as I used last time. NGK DR8ES-L.  I'll try DENSO X24ES-U later on after road tests with existing setup.  Today freezing and snowing...

I have felt that the coils become warm when ignition is on, but that is normal, right?

I started it again later on to show my friend visiting me.  (Biker friend since 35 years. He never stopped driving bikes as I did between end of 1990 until now)
The engine ran even better 2:nd time NO problem with black smoke as before. Quick throttle response without any hesitation. Fair idle between 1400-1600   No sign of flowing floats. Neighbors did not complain or called Police either... 

I'm convinced that I can update the main jets with one step bigger, 120. I must compensate for warmer plugs.
 I had almost decided to switch cam and save the CX-7 with the growing pile of parts that will be my next CB750 project with bigger bore,  900 or 915.

- Anyone that had to use one step warmer plug? Caused by tuning...
836cc, ported head, 34mm inlet valves, CX-7 cam, lightened alternator, 4-1.
Different now compared with many years ago running normal plugs: cam and pistons NOS RC cast 836 that need opened chambers in head as My head has.

[Medyo Bastos]

i gave up on ngk plugs quite a while ago. i been running the autolites with much better results!


-save a tree, eat a beaver using Tapatalk

[PeWe]

i gave up on ngk plugs quite a while ago. i been running the autolites with much better results!


-save a tree, eat a beaver using Tapatalk

I cant fine those plugs in Europe, only USA sites.
- Which plug model do you use?
The model you found to work well, might be warmer than NGK 8 something....

[Don R]

My dragster's engine came with 10's, it idled poorly when cold and was difficult to warm up. It ran OK after it warmed up. I put in 9's and it's been great since then. Hotter plugs can do the job.

[MRieck]

Just curious...did you time the cam?

[PeWe]

Just curious...did you time the cam?

- My engine?
Yes, timed during loong time when doing it for first time facing the play in valve train affecting the result.

I've read some threads about hot cams and rich on low rpm's, mostly car forums. Advance the cam, IN and EX earlier will help rich on low with a hot cam?
(Closing the inlet valve earlier)
I'm in the order phase of drain screws for the VM29 carbs. Make clear tube test on all 4 simultaneously to SEE the fuel level in float bowls. It might be wrong despite measured floats on the bench. Floats sticking to the float bowl, float valves etc. Too low is not good either.

[Turbogrimace]

Definitely start with the float height. I was fiddling with tuning them a bit under the assumption that the float heights were close... They weren't and they were all high.  After I adjusted them, the rich condition was gone. I was significantly closer to tuned in.  Your engine is very similar to mine so your jetting should be close. 

I also notice that you swapped your needle.  What is the difference between the -06 and -08? This question is better suited for the other threaded we've been working on about vm29 tuning.  I'll copy your answer and put it over there for better documentation.

[PeWe]

Hi Turbogrimace,
The needle is VM29 standard for CB750.  Everything according to SUDCO manual.
I have only changed pilot and main jets.
0.6  was shipped for Kawasaki and Suzuki.

The warmer plug is my plan B since I remember a friend on a tuned Kawasaki Z1-1000 that solved the fouling problem for good with warmer plugs.

I'll check the fuel level in float bowls as soon as the tool arrive in my postbox. http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Fuel+Level+Gauge+Tool+for+VM29+smoothbore+carbs&item=ZSM-VM29FT
Too many days waiting for parts the last year....
After that I know if next step advancing the cam is needed.

I hoped that other should answer about D7EA plug, that they use that plug too without problems.
I have several sets with new plugs of std CB750 heat range. D8EA, DR8ES-L and DENSO X24ESR-U which i want to use without fouling them.

[Doctor_D]

First of all, ignore the Sudco manual.  The correct needles may be nowhere near what they recommend.  The best way to get the correct answer is part-throttle pulls on a dyno.  To get my RS's dialed in, we did 1/4, 1/2 and 3/4 throttle pulls from 3-6,000 rpms.  I also like to do a just barely cracked-open 2-4,000 or 5,000 rpm run for slide cutaway, pilot and air or fuel screw tweaking.

And, I wouldn't go hotter on the plugs for a high compression 836.  I had to go colder to get my advance-mark at the right place on the plug's ground strap regardless of how much or little timing I ran it with (about half way up the ground strap is the rule of thumb).

[PeWe]

David, thank you very much for your information.
-Do you have jetting/needle examples for the VM29 and CB750? (Rather common question in the forum)
I bought my VM29's new from my local tuner 30 years ago. No issues with fouled plugs then. Might be a leak when rather big jets were needed. 25/127.5. Better vacuum now since I adjusted the carbs to fit OEM carb boots.

[01Thomas]

Hello PeWe
To get back to your original question: a plug with a hotter heat range means that the plug retains more heat, while a plug with a lower/colder heat range dissipates more of it's heat.

A hot plugs' heat is very localised - it really only affects the carbon build-up and burn-off on the porcelain centre electrode. A hotter plug will not increase engine operating temperature significantly. The engine is still cooled by air cooling the fins the fins and the oil. Standing and idling at a traffic light for five minutes will have a more detrimental effect than going up a heat grade or two on the spark plugs.

Of course you could take it to extremes, but one or two heat ranges hotter should be fine.

You may well feel the engine is running hotter with the hotter plug, but that's more likely to be because you can run the engine harder now that it's not fouling it's plugs.

Regards

[PeWe]

01Thomas: Thank you very much for your input. Warmer plugs seems to be rare in a CB750.
I'll continue with carb jetting, floats, eventually more adv cam to avoid fouled plugs.

[PeWe]

Update: I'll switch back to std plugs such as Denso X24ES-U when the carbs are much better.

Floats WERE wrong as several guys here had told me. I had to wait for tool checking it....
Clear tube tool helped to adjust all 4. After that smaller pilots and much better idle. Not so rich smelly fumes. No visible smoke when warm despite running the engine outside, temp below 0*C.