compression test results

[neat street]

Hi All, I have just completed a compression test and found that the results are, 1-2-3 test gives results of 138psi. no 4 is at 150psi, after placing oil in the head of each, the results go up to 175psi (test done with engine hot). I have checked my Climers manual, and although it tells how to do the test, it does not give what the results should be (except + - 10%)? the problem I have been having is coked up plugs, and trying to determine what the problem is (even though it does not appear to be oiled up plugs), thought it best to check anyway as this is the easiest test) I have also moved the clip on the carbs to the highest setting, and this does not seem to have solved the issue? any thoughts/help would be appreciated.
many thanks
John

[Spitfire]

Hi John, what bike is it, has it got a standard exhaust system, is the air filter in good condition ? The reason I ask is that with an after market silencer fitted (standard downpipes) my CB750F1 was fouling plugs, the plugs/coils/plug caps were new, the air filter was new, the carbs were clean and jetted as per new, and the air filter was new, so I drilled a couple of holes in the AM silencer and now all is well.

Cheers

Dennis

[neat street]

Hi Dennis, thanks for the reply, Bike is same as yours, 1976 F1,  have new filter, new jets (as standard), new coils, etc, tried hotter NGK plugs, balanced carbs, played with height settings on the needles (now at highest point), new plug caps, and new exhaust (stainless 4 2 1 from US) no silencer box, just a small cone in the end (should make it leaner).
Thanks again for your reply
John

[Spitfire]

I also tried different plugs and the best were ND X-24ES-U as recommended by Hondaman on the US site, I've stayed with these ever since.

Cheers

Dennis

[K2-K6]

John, which number NGK plugs did you go from and which did you end up on?

[neat street]

Thanks for your reply's, the plugs I have tried are:- NGK DR8EA, BOSH XR465, and the hotter plug NGK DR8ES. I am at a bit of a loss, I don't know if the compression test results would cause the fouling, or if they are acceptable? I will try some of the plugs that Dennis recommends, but if anyone has other possible solutions, that would be a great help?
many thanks
John

[K2-K6]

It's the number of the plug you need to change, for example the NGK needs to go down to a 7 to swap the heat range.

For these engines NGK 8 is the most normal for general use, going down to a 7 would give a plug that stays hotter during the same running conditions and would be considered the optional choice if ambeient temp is consistently low and engine loading was fairly gentle. The opposite of course is higher ambient and thrashing the motor, you'd head towards a 9 grade to prevent the plug overheating. These are confusingly transposed sometimes in the way people express if you need hot or cold plugs.

A NGK-8 would have an equivillant of ND-24 and a NGK-7 would have a ND-22 as an equal

Although it's by no means a rigid level, Honda manual suggests ambient temp of less than 5C/41F as a start point for dropping to NGK-7 plugs.

It may be worth trying it on the 7 or ND-22 to judge what you get.

[neat street]

Hi K2-K6 and Dennis, thank you for your help, I will try the hotter plugs from Denso, and will let you know how I get on. They wont arrive till maybe the end of next week, so bare with me, if this still cokes up the plugs, does anyone know if the range I am getting with the compression test is OK? if not, then engine stripdown again, but would be ready for the warmer months? I will also double check the timming and mixture, but I am sure they are OK. the only other option is maybe going for different size jets? standard at present.

[K2-K6]

As far as I can see the compression psi should be in the range 150/175.

If you take a Honda figure of 175 and a tolerance of -10% then you've got approx 157ish as a general aim point.

Your readings don't seem too far off and I'd expect to be a fair bit lower if they were to give you serious problems (probably heading down to around 120) it doesn't seem at the moment to push you to a strip down without further investigation.

Did you have all the plugs out when you measured the compression... as it spins over faster. lso did you hold the throttles full open when you did it too?

Just a quick question about the needles, do you have the clips right at the top of the needles so that the needle drops down as low as possible in the slide?

[neat street]

Hi K2-K6, yes, needles are in the top position on the needles, and the engine was warm when tested, all plugs taken out with the throttle wide open. I am glad you think the rings/barrel may be OK (will hopefully save the engine taken out again), but still bemused as to the problem?? any other ideas would be a great help, after I try with the new plugs, if this doesn`t work, anyone with other suggestions would be great (maybe look at jet sizes?)
Many thanks for your help, always appreciated.
John

[Spitfire]

Carb info

Honda CB750F – F1 Carburettor

Main jets    #105
Idle/slow jets   #40
Air screw opening 1 turn +- 3/8th turn
Cutaway number 2.5
Valve seat diameter 2mm
Fuel level 26mm
Jet needle setting 3rd Notch


Cheers

Dennis

[neat street]

Hi all, just tried the new plugs, ran perfectly for a while, although needed longer on chock than before, and does not like the new set-up too much (highest point on the needles). so I am guessing that the next point down on the needles is best? also still having coke problems using the high temp Denso plugs. Any thoughts as to the pressure test result, or other thoughts that could stop/cause the plugs to be coking? I am a bit stuck now.
all help appreciated
Thanks, John

[Spitfire]

Hi John, it sounds like a back to basics job, going through all the basics, timing, carb settings etc, pretty boring stuff but the answer must be there.

Cheers

Dennis

Just had a thought, this may help it is the thoughts of Hondaman from the US site on the subject of plugs

"Today, I cannot even find the D8ES-L or the X22ES-U or X24ES-U plugs: they have been replaced with "broad range" units like the D8EA. It's too bad, really, because I notice the extra missing pony or two. I had to lean out the jetting to run with these, so they will stay clean. I ran 105 mains and 24mm floats for years with the older plugs: I have to run 26mm floats and 100 mains with the new ones or the plugs will foul in 1 tank of gas.   

The closest match to that plug today is the ND X22ES-u. This plug is approximately a "D7.4E" heatrange, where the extended-tip "-L" series plugs ran at D7 temps on their tips, with D8 heatsinking in their seals. The ND plug has a thinner, more extended tip, similar to the "-L" plugs.

I normally run the X24ES-u with regular gas on the highway (midgrade with heavy loads) and the X22ES-U on midgrade for commuting work, when there's lots of idling going on, to keep the plugs cleaner. In cold weather commuting, I use the X22 with regular gas to make it warmer and cleaner.

The D8E series is too cold for lots of idling (it fouls after 1000 miles or so), the D7E series too hot for highway use. Honda specifically asked NGK for this plug, in 1970.

The "-L" disappeared from NGK production in the mid 1990s. I miss that plug".

I got my plugs here: http://www.camskill.co.uk/m18b0s0p0/NGK_Spark_Plugs_and_Denso_Sparkplugs

[neat street]

Thanks gain for your help and input Dennis, much appreciated. as you say, look like back to basics.
many thanks, and have a great Christmas
John

[Spitfire]

Actually John you did me a favour, I could not get the ND X22ES-u plugs when I first restored my bike but you got me to look again and I have just bought some, so will try them out.

All the best

Den