Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: mgy66 on July 13, 2015, 06:51:59 PM
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Hi,
I have my 750K1 pressure gage tripping after 15min usage (London commute exclusively). The bike was rebuilt with a 850 cc kit. I have checked the oil temp from the side tank... 82C/180F to 92C/200C. I have put the bike to factory spec (jets, filter box, exhauste 4 in 4, air screw I tried 1 and 1.5 turn).
Anyone has a measure in similar condition (UK is not specifically a climate for high spec oil)...
I have given it for check and oil replacement. My workshop specialist said the oil pump is fine (I suppose he has also checked the valves) and refill with standard 10W30 semi-syntetic, but still tripping after 15min.
I have noticed the earlier discussion on oil selection. Shall I try a more high temp mineral oil? What would be the reference, I always mix up the grades...
Could it be the oil pressure/temp gages?
Thanks for helping
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Air cooled bikes always get hotter with hotter weather, especially with London temp.
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Factory spec on jets etc with an 850cc kit fitted will leave it running lean. Never done it myself but it will need rejecting. Graham will probably have more idea than anyone.
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Humm... i actually rejetted up... i had upscaled the #40 by drilling at +50mic with main at .120 with velocity stacks bu had black spark plugs like running too rich. I thought that was the cause of my high temp (too much fuel washing up the combution chamber oil film and/or contaminating the oil somehow) and put everything back to standard, jets and filter box and new filter.
Despite standard set up (#40 and 105) on 850cc i have rechecked the plugs and again black as if still too rich... i actually can't get plugs to turn into clean or even white. Needles seems to be fully engaged, only tip is visible from top of the carbs.
I also checked the floats at 25mm from seal to bottom of the float when up (valves closed position). I was told that running with high level would make the mix too rich.
So would be keen to have some measure of oil temps... Is 93C/200F critical? Shall i top up the oil tank in xs of the high level?
Cheers
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I can't guarantee how accurate this is but an oil blender once told me 130c was the upper limit for mineral oil, with 90 to 110 being ideal. If temps don't reach 90 or exceed 120 they recommended more frequent changes.
I've only monitored an air cooled single, 85 was the norm, rising to 120 in traffic on a 27c day.
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Hi
Some input on the jetting for big bore - I'm running a standard compression 836 kit (Cruz Image kit from Japan) on std airbox/exhausts (on my K2) and plugs a nice sandy colour on 110 main 40 and 1 1/4 air screws. Needles are on middle setting. It runs great throughout range (not revved it hard yet as only done 400 miles). Only thing out of the ordinary is it only needs 3/4 choke when starting from cold and needs backing off almost immediately.
Phil
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How is the old girl Phil. Not heard about it for quite a while.
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Roy
She's Great! Done about 400 miles and no issues at all except those soggy old springs mentioned in other post. Have sent for a set of progressive springs. She's slower than I remember but that's probably because I'm stepping off a ZZR1100! It's very smooth and I enjoy driving it slow or around town as it's a light weight compared to the ZZR. Only other issue is front rim is slightly out of true but I'll get that sorted with the guy who built the wheels this winter.
Considering I built it from scratch I've had no teething problems - so well chuffed!
Phil
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Sounds like the K2 is running sweet Phil, plug colour is always a good sign.
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Good to know...
@Totty, Definitely within the 90c to 110c, rather lower bound, semi synthetic oil, traffic jam / commute in town with loads of stop and start at red light. So could come from the oil pressure gage or may worth upgrading the oil to higher viscosity. My worry is really that the pressure drop would result in engine failure or bearing failures...
@Philward, I am not so far of in spec, only running 105 main and needles seems fully engage so will double check that again. I have noticed gap at opening the throttle which seems to indicate lean mixture. Yet my plugs are really black. Will take a picture and share the colour. What would be the result of miving the needles up? Richer i suspect which is what i want to avoid
Thanks
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It's not the oil breaking down as it still did it with fresh. I don't think it's overheating at those temps, I also don't think it can be oil dilution with fuel, so if the mixtures out that may be a separate problem. It sounds like worn bearing or pump, or an inaccurate gauge.
There's a chart that I don't have to hand which shows oil viscosities against ambient temperature, I'd try the thickest that's ok for the current temperature range and see if it made a difference.
When are you checking the plugs? Remember it could be really lean at one throttle opening and really rich at another, it could also be darkening them with a bit of burnt oil.
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Yep, i have ordered draper oil dainage tank and will look at oil chart. Hope it is not the bearings as that can be misery ahead. Is it easy access to replace inacurate gauge, i.e. does it requires opening the engine?
Good point on the dark plugs. The colour is rather mat than shiny so thinking more toward rich mixture than burnt oil. Meanwhile, you are right, difficult to say where on the curve throttle is mixture out. All i can say is that i use the bike in london so rarely more than 30mph, loads of stops and goes and half an hour commute. I usually check the plugs after a commute.
What about carbs levels, can it affect the mixture on the lower part of the throttle curve? I have also noticed idle can increase to 3000 rpm without touching the throttle, forcing me to adjust idle screw down at red lights and then up when it revs go down to avoid stalling at the next red light.
Painful... ::)
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Wondering idle can be an air leak at the inlet or carbs, this will also affect the mixture so it's worth looking at before looking at float heights.
I don't know the 750, but on the 550 the pressure switch is easily accessible, it screws into the top of the pump.
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Ok, so this may be of interest for oil visco. I will probably go for 15w40 which i think is easily to source.
http://www.ratwell.com/technical/OilSelection.html
Here attached two of the 4 plugs...
Re air entries, i have new inlet silicons from the airbox and a new air filter. Can it come from carbs top gaskets where the needle are being pulled up?
Thanks
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Spray a bit of something flammable (wd40) will do on the carbs and inlets a small area at a time listening for a change to idle. If it only wonders when warmed up them it'll need to be warmed up for the test. If the idle changes you've found a leak.
I'm not sure if a leak at the carb tops would be that noticeable, the problem is generally on the engine side of the carbs.
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as far as oil temp is concerned,I looked on the WWW for air cooled engines,
and seems they can run at upto 260F/or about 130C,
did you check the head?
or sump?
or oil tank?
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I will test after between carbs and engine but there is only the black silicon tubes so if air leakage it can only come from loose rings around the silicon tube.
Temp is measured at tank side 200F.
I think i read in haynes book that the gauge can trip at idle but here we are talking from 1000rpm to 4500rpm...
I am going to test few options before changing the oil... get the idle steady with search of air entry, move to 110 main jet and increase the air screw to balance air mixture at idle and pick up, then change the oil spec and if it still tripping change the sensor, hopefully get one spare from DSilver...
Thanks to you all...
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Sorry got to ask just what you mean by gauge "Tripping"
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Think he means the oil pressure warning light coming on Bryan. Been watching this thread with interest.
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;) lol lets try and kick this one in the long grass
about two weeks ago i came back from a half hour blast at 70-80 mph up the m4 we had been to a bike show and crap weather so was not hanging around, be that the bike(cb750k0) was a rough rebuild and the oil light would flash on at tick over
by the time we stopped i showed my mate that the oil light was staying on up to 2000 rpm ,he said the mains must be knackered, i said yes probably but it still goes well and no nasty sounds, i will change the oil switch when i get home
and yes the switch was giving a very low reading,and the oil was very hot and thin ,after checking with a pressure gauge (40 psi)and a new oil switch it still goes well,ofcourse there is wear there its 45 years old, but the oil light dont come on anymore ;)
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So Steff, you are suggesting replacing the oil switch rather than the oil...
I have moved up the mains to 110, raised the air screw to 1.5 turn, still running hot and thin oil...
Did not try yet the dw40 but noticed that there is a bit of gap at the junction of the needle holding screws and the support plate .. that may allow some needles to move up and down with instable idle. Will tight it up.
I was to try 15w40 oil instead of 10w30 before going to replace the pressure gauge but Steff, i am curious to see how you can measure the pressure... where do you insert the pressure gauge? Picture of the location of pressure gauge would help. For some reason i understand it is inside the engine in the pump...
Cheers
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its easier to change the oil switch first, unless you need to check the actual oil pressure,if you already have hold of a oil pressure gauge, you will need to remove the plug at the end of the oil gallery,that's where the oil switch fits but that's on top of it
the pressure gauge goes into the oil gallery just above the points cover take a look at some chopper photos a lot of custom bikes use a oil pressure gauge on their bikes
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Steff, thanks for the input... i think i know where is this switch. It must be the black connection in the middle of the gallerie right above the starter cover. I have checked on youtube and it seems to be a classic issue.
www.youtube.com/watch?v=MhQeIY_gTOg
I dont know why the repair shop did not suggest it when he checked the pump and changed the oil... or i hope it is the switch and it is easy to source on ebay or DSS.
Cheers