Topic: difficult cold starts/ fine tuning the 718

[paulages]

for those that don't know the engine, it is a cb550 bored to 718cc with a megacycle 126-20 cam, stage 2 porting, cb750K carbs, K&N pod filters, and free-flowing exhaust. the plugs are D7EA.

the dyno shows a nice A/F mixture at idle, and i'm continually fiddling with the mixture screws for the best off idle response. they seem to be happiest at 1.5-1.75 turns out. it isn't happy idling low, but without a tach i can't say exactly where that is. by ear, i'd say smooth(ish) idle is 1500-1700 RPM. i can live with the poor idle, as is runs beautifully everywhere else, but starting in cold weather is making me really unhappy. the electric starter is plugged off, and kickstarting can really be a #$%* unless i resort to starting fluid, at which point is starts easily and warms up fine. after that, it usually starts first or second kick unless i let it sit for a day or so.

priming the cylinders with a few kicks WOT, full choke, and kill switch off usually helps, but sometimes it just doesn't want to start. any ideas here? i hardly want to tune the idle circuit for cold starts, but it really impedes my ability to ride in cold weather. i've had cb750s that wouldn't start cold if the idle screws were 1/2 turn wrong, but i've tried everything in 1/8 increments.  

[Gordon]

What kind of ignition are you running?

[paulages]

What kind of ignition are you running?

i knew i'd leave something out.. 

Dyna 2000, usually set on the third advance position at 37* total advance.

[Gordon]

Have you tried different advance settings?

[TwoTired]

I think it would be a mistake to tune the idle circuits for cold start.  That is the job of the choke, to make the vacuum in the carb throats so high as to draw more fuel from any fuel delivery circuit open to the carb throat area.
Though not exactly desirable, choking should actually be able to flood the cylinders way over rich (which is also outside the ignite range).
Which year 750 carbs are you using?  Do they have acell pumps?
Have you looked at the choke plates for full closure or bypass holes? (I don't know the early 750 carbs that well, only principle of operation.)
Have you pulled plugs during non-start conditions to see if you are flooding or starving during start conditions?
What kind of vacuum does your cam pull at idle?  Do you know the cam timing numbers?  Hot cams typically have poor idle vacuum. (trade-off)
What is you idle speed setting, warmed up?  Can't you hook up an electronic tach to get this? (An RC airplane/helicopter tach can probably be adapted for this task.)

Have you checked the voltage that coils receive during start up?
What coils do you have?
Does it start easier with the headlight off?
What spark gap have you selected?

Last thought is a restrictor "plug" for the exhaust, just to help with starting. (If none of the above yields happy results.)

[turboguzzi]

what about spark plug caps?

some think that 0 ohms caps is a good thing but engines run better with 5-10K ohms..

[paulages]

thanks for answering TT!

the carbs are early K carbs, from an unknown year. K3 probably- no accelerator pumps. to begin with, i'm up against difficulty at idle with the cam and filters. if i could fabricate an airbox to enclose velocity stacks like the cb750 airbox, i'd do it. i still think about doing so, but the position of the frame makes it difficult.

the choke plates do close fully. the plugs do not look very wet during the "no-start" condition, but the fact that i can never use full choke once it starts tells me something about the general richness of my idle mixture, i think. i think a post-break-in carb resync might be in order in any case, but when i synced the carbs last i believe the vacuum was in the 18-22hg range.

the dyna 2000 does have a location for an electronic tach. i need to find a way to utilize this, at least for tuning. i'd like to be running a tach eventually anyway. i believe it was around 1500 RPM on the dyno, but i fiddle with the adjustment screw every time i warm the bike up.

i haven't checked the coil voltage during start up. my headlight is always on, and maybe this is an excuse to rig an on/off switch i've needed anyway. i do suspect the ignition could be unhappy with its starting voltage, but the spark looks nice and strong when testing the plugs.

the spark gap is at .028" or so. the exhaust plug is a pretty good idea, though about as impractical as carrying around a can of starting fluid if i'm not at my shop... on the dyno there was no power increase at all when the baffle was removed, so i suspect i could induce a little more back pressure permanently without sacrificing top end.


i'll try a few of these suggestions and report back... thanks!

[paulages]

what about spark plug caps?

some think that 0 ohms caps is a good thing but engines run better with 5-10K ohms..

that's a good thought. i realized i didn't answer TT in regards to which coils i'm running anyway, so i'll ask you this: with my 2.2 ohm coils needed with the dyna 2000, would adding resistance at the cap be bad for the unit?

[B.O.X.N.I.F.E.]

Wow, can the 550 charging system really handle 2.2 ohm coils?

[turboguzzi]

no resistance at the caps/spark mean spark is very short.

caused erratic running in a friend's bike.

caps with resistance should have that written on the bakelit

TG 

[Bill/BentON Racing]

Paulage,most 750's using these carbs didn't like full choke once they fired,give us carb #,we can tell what year,I would definitely wire separate headlight on/off,try the plug caps,maybe .035 gap,let us know what you find,GSXR 750 1986 tach wires up easily and reads to 13,000 rpm and has 11,000 to 13,000 @ 12:00 also outlined in red(right where you want to be)Ok close?,Bill

[TwoTired]

Don't confuse primary resistance with secondary resistance.  They are isolated from each other.

I really don't know much about how the dyna 2000 operates.  So, I can't guess the duration the coil current exists per revolution.  This is the dwell spec., (which is 195 degrees of crank rotation for the points system) and determines how much total draw the coils/ignition has on the battery.
All the current draws headlight coils etc. will lower the battery voltage during start operations.  But, since I have no way of knowing what the dwell is for the dyna 2000, I can't estimate the Vdrop, or what effect, if any it has on the ignition.  
I'm kind of surprised that the Dyna 2000 system doesn't specify a larger gap at the spark plugs.  If it is a coil discharge based system (Kettering), you won't realize any benefit of higher voltage potential without a larger spark gap.

As for the secondary, you really should have some resistance in either the spark lead, the plug cap or the spark plug itself.
Does the dyna power pulse the coils for the spark event, or does in discharge the coils for the spark event?  The stock points system and the Dyna S both discharge the coils for spark.
If it works like other systems, then the coil discharge collapse will still build until the spark leaps the gap, (whatever voltage it needs) the secondary resistance will slow the coil discharge current, and delay the drain time, creating a longer spark event duration.  This should allow a longer time to ignite a mixture and the atmosphere near the leaping spark should get hotter during cold start, making it more likely to ignite an less than ideal mixture.

What wattage headlight are you using?
Can you remove a headlight fuse easily for starting?
What is your battery rating/type?

Any schematics or theory of operation available for the dyna 2000?

The 750 PD carbs have a spring loaded door in the choke plates to relieve some of the vacuum when the plates are fully closed.  They also have a hole in them where the accell pump squirters shoot through.  Are the choke plates on your carbs solid?

If you don't have run issues when the engine is warm, I still don't think you should mess with the inlet restriction, just to make it start easier.
You have to determine there is reliable spark under compressed gas conditions, and that your cold enrichment capabilities are adequate.
But, there is a possibility that the cam may simply not create a high enough vacuum for easy cold starting.  Dry plugs seem a damning culprit.

By old BSA had "tickler valves" that basically let raw gas into the inlet runners for cold starting.  No problem getting the plugs wet if you were overzealous with their use.  Push the button 'til you smelled gas, and giver 'er a kick.

What position do you hold throttle for start up?   The more closed the slides are, the deeper the vacuum seen at the pilot circuit outlet.  Of course that also leaves the throttle valve at minimum opening for fuel delivery.  But, if you have a low vacuum condition when the slides are open, the jets aren't going to flow fuel regardless.  If you have little inlet restriction, holding the throttle open will reduce the throat vacuum and fuel flow accordingly.

[754]

How cold ? This here 750 is not bad, until low 40s or colder, then it starts harder. I will switch to 10-30 so it kicks easier, and runs warmer.

 Get a squirt bottle for gas priming, I doubt starter fluid will do your motor any good..

[Bill/BentON Racing]

754 right about starter fluid,detonates easily,use what you're trying to get in there in the first place,raw fuel,like he said squirt bottle etc.,Bill

[paulages]

wow, lots to get to. first off: i can't tell you how many times i've wished this bike had a tickler like my BSA. i hate using the starting fluid... not only is it hard on the engine, it probably doesn't do much good to the filter oil. good sense on the wire caps- i just used what dyna provided in the dyna2000 kit. they aren't really caps, but rather rubber protectors for the ends. i'll pick up some NGK caps tomorrow.

i'll get to TT's post in a bit.. his usually require some homework. 

[B.O.X.N.I.F.E.]

Side note, if you want a headlight kill switch the 74 550's have them. I use it a lot, when I was on my trip I would kill the headlight and charge up my cellphone on long stretches of highway during the day. Use it too when the bike is at idle.

[paulages]

Side note, if you want a headlight kill switch the 74 550's have them. I use it a lot, when I was on my trip I would kill the headlight and charge up my cellphone on long stretches of highway during the day. Use it too when the bike is at idle.

Thanks. I have a '74 switch housing, but i'll probably just wire an on/off toggle in the dash somewhere.

[paulages]

ok, i'll try to answer a bit of this... i need to get the bike back on the stand and do some analysis, which will take me a couple days to find the time for.

Don't confuse primary resistance with secondary resistance.  They are isolated from each other.

yes, this makes sense. i just wasn't thinking it through.

I really don't know much about how the dyna 2000 operates.  So, I can't guess the duration the coil current exists per revolution.  This is the dwell spec., (which is 195 degrees of crank rotation for the points system) and determines how much total draw the coils/ignition has on the battery.
All the current draws headlight coils etc. will lower the battery voltage during start operations.  But, since I have no way of knowing what the dwell is for the dyna 2000, I can't estimate the Vdrop, or what effect, if any it has on the ignition.  
I'm kind of surprised that the Dyna 2000 system doesn't specify a larger gap at the spark plugs.  If it is a coil discharge based system (Kettering), you won't realize any benefit of higher voltage potential without a larger spark gap.

I have none of the dyna 2000 specs other than what was provided with the kit, but your comments provoke me to want to call them for more specs and a chat about these points. I'll look back through the literature, but I don't believe they specified a larger gap, though it does make sense. I estimated mine above, and are probably actually just under .030".
[/quote]

As for the secondary, you really should have some resistance in either the spark lead, the plug cap or the spark plug itself.

again, I just installed the kit the way it came, wires included. The dyna kits come with 2 wires, the plug ends made up already, and when you cut them in half, you make up the coil ends. The 2000 kits come with suppression core wires.

Does the dyna power pulse the coils for the spark event, or does in discharge the coils for the spark event?  The stock points system and the Dyna S both discharge the coils for spark.
If it works like other systems, then the coil discharge collapse will still build until the spark leaps the gap, (whatever voltage it needs) the secondary resistance will slow the coil discharge current, and delay the drain time, creating a longer spark event duration.  This should allow a longer time to ignite a mixture and the atmosphere near the leaping spark should get hotter during cold start, making it more likely to ignite an less than ideal mixture.

As far as I know, it is a discharge system. I feel like if it was a pulse-fire system, dyna would advertise this loudly. I could be wrong though.

What wattage headlight are you using?
Can you remove a headlight fuse easily for starting?
What is your battery rating/type?

the headlight is a stock honda lamp, 30/50 i believe. I'll have to check to be sure. I did try starting with it disconnected today with no better results.

Any schematics or theory of operation available for the dyna 2000?

I'll attempt to contact them tomorrow for more information, and look for the literature that came with it.

The 750 PD carbs have a spring loaded door in the choke plates to relieve some of the vacuum when the plates are fully closed.  They also have a hole in them where the accell pump squirters shoot through.  Are the choke plates on your carbs solid?

my carbs are not PD carbs. I'll look at the stamping tomorrow, but it doesn't really matter... they're K1-6/ F1 CB750 carbs. jetting does not match up with a particular year within that range.

If you don't have run issues when the engine is warm, I still don't think you should mess with the inlet restriction, just to make it start easier.

I agree, and this was my initial thought. like i said, the dyno was happy with my carb settings warm. I can't help but suspect weak ignition. when it does fire, it just catches and is suddenly strong.

What position do you hold throttle for start up?   The more closed the slides are, the deeper the vacuum seen at the pilot circuit outlet.  Of course that also leaves the throttle valve at minimum opening for fuel delivery.  But, if you have a low vacuum condition when the slides are open, the jets aren't going to flow fuel regardless.  If you have little inlet restriction, holding the throttle open will reduce the throat vacuum and fuel flow accordingly.

you know, even warm the throttle position is a little tricky on startup. usually if i let someone else ride it, they'll fumble with the start, and i'll hop on and fire it up first kick. i usually pull the throttle cable taut, and then just lightly open it as i'm kicking it through.

sometimes it just feels like i can't kick it through enough with one stroke of the kickstarter. I love how on my BSA, if the kickstarter is at the top of a compression stroke it feels like you're really spinning the engine fast. the 550 feels like the 1st gear kickstart ratio just isn't tall enough. at least on mine anyway...

[paulages]

i've been thinking about the resistor caps, and i'm concerned about one thing: without knowing the dyna's internal specs, could there be any risk to introducing more resistance in the high-tension circuit? my thinking is that said resistance is between the coils and plug and shouldn't affect the "brain", which is located in the primary circuit, but i'd hate to do anything to void warranty. i'll try to call their tech dept tomorrow, but it seems strange to me that they wouldn't provide the optimal components since it's sold as a kit.

[paulages]

all i can find on dyna's website is the installation instructions here: http://www.dynaonline.com/skins/downloads/instruct/DDK1-5_2-1_3-2.pdf

you'll notice that it refers to "an innovative dwell control scheme [that] maximizes spark energy all the way to 16,000RPM..." the wires that came with it are in fact suppression core, but i can't find specs on them. i'll take an ohmage reading when i'm at the shop. i suspect they provide adequate resistance. no mention of plug gap.

[paulages]

ok, i had a chance to do a few tests. first off, the suppression core wires provide plenty of resistance (about 5,500 ohms). plugs were in fact set at .028". i will widen the gap. i am running D8EA plugs due to having a higher compression engine... maybe i'll go back to the D7EA plug that the 550 and 650 prefer. also, the inner two plugs looked much leaner than the outer two. i've been thinking about building an airbox for this that would contain velocity stacks, much like the cb750 airbox design, but with a larger internal area. at the very least, i'll probably richen those two soon. still, the apparent difference in running conditions between the inner and outer cylinders is probably at speed, not at idle.

i did compression tests that were a little worrisome. accurate readings are a little difficult with only a kickstarter to spin the engine, but i got 150 PSI on cylinders 1, 2 and 3, and only 90 PSI on #4. pressurizing each cylinder showed a perfect seal at TDC on the first three, and ring leakage on #4. after this finding i did another compression test and got 150 PSI. still leaks with pressure though. guess i may have to re-hone and maybe re-ring that one soon, but i doubt it's the cause of my poor cold start.

[TwoTired]

Most of the SOHC4s run better cold with the hotter heat range plug.  But, I really don't know if it makes a difference initiating a stone cold engine.  Worth a try and it's a pretty easy test.

still, the apparent difference in running conditions between the inner and outer cylinders is probably at speed, not at idle.

Certainly could be right.  Unfortunate, the lack of supporting data.  But, it is a reasonable assumption given that all carb aren't fed from the same pool of air pressure when air moves around bike and rider.

Where can I find good pictures of your bike with details of the area behind the the carb inlets.  Do you have a rear inner fender?

So, I guess you are happy with the coil input voltage during kick start?

[mlinder]

ok, i had a chance to do a few tests. first off, the suppression core wires provide plenty of resistance (about 5,500 ohms). plugs were in fact set at .028". i will widen the gap. i am running D8EA plugs due to having a higher compression engine... maybe i'll go back to the D7EA plug that the 550 and 650 prefer. also, the inner two plugs looked much leaner than the outer two. i've been thinking about building an airbox for this that would contain velocity stacks, much like the cb750 airbox design, but with a larger internal area. at the very least, i'll probably richen those two soon. still, the apparent difference in running conditions between the inner and outer cylinders is probably at speed, not at idle.

i did compression tests that were a little worrisome. accurate readings are a little difficult with only a kickstarter to spin the engine, but i got 150 PSI on cylinders 1, 2 and 3, and only 90 PSI on #4. pressurizing each cylinder showed a perfect seal at TDC on the first three, and ring leakage on #4. after this finding i did another compression test and got 150 PSI. still leaks with pressure though. guess i may have to re-hone and maybe re-ring that one soon, but i doubt it's the cause of my poor cold start.

Time for the patented VELOCIPOD!

http://forums.sohc4.net/index.php?topic=20869.msg645658#msg645658

[paulages]

Most of the SOHC4s run better cold with the hotter heat range plug.  But, I really don't know if it makes a difference initiating a stone cold engine.  Worth a try and it's a pretty easy test.

still, the apparent difference in running conditions between the inner and outer cylinders is probably at speed, not at idle.

Certainly could be right.  Unfortunate, the lack of supporting data.  But, it is a reasonable assumption given that all carb aren't fed from the same pool of air pressure when air moves around bike and rider.

Where can I find good pictures of your bike with details of the area behind the the carb inlets.  Do you have a rear inner fender?

So, I guess you are happy with the coil input voltage during kick start?

I can't say I'm totally happy with coil input voltage. The battery at rest is at 12.43V. With the switched power on, there is 11.38V at the coils. With the tail and head lights on, it all the way down to 10.78V.

I'll try to find a picture for you when I get home, but I'm not at my home computer. I do have the rear inner fender. the tank sits pretty high, and the area around the filters is quite open.

[Bodi]

Interesting that you're tuning for proper a/f ratio at idle. I've understood that the non-pumper carbs like to idle quite rich so they don't stumble so much on throttle.

[paulages]

Interesting that you're tuning for proper a/f ratio at idle. I've understood that the non-pumper carbs like to idle quite rich so they don't stumble so much on throttle.

i'm not- i've tuned for best off-idle response, which is a hair richer than the best idle position.

[paulages]

TT- this is the closest picture i could find with the carbs installed:



my idea for the airbox is basically to build an aluminum box to surround velocity stacks, much like the cb750 airbox, but with an air chamber/filter in the rear triangle like the 550 to allow maximum air flow. that area is wide open, so i can make it as large as i want to allow lots of stable air. constructing it so that i can get the carbs on and off with ease seems the real challenge...

[TwoTired]

Hard to picture where the legs are when riding.  Those are going to effect air flow, too, of course.
Not too hard to see where the inner carb inlets might get higher pressure than the outers, though.
Given any thought to adapting the stock air plenum? (I know, heresy...)

I suppose none of this has any relevance to start up issues, really.  If your choke plates don't give enough enrichment, there's really no point in making the idle circuits richer just for cold start.

Start procedure suggestion:
With the choke plates closed give'r wide open throttle for a kick or two after you know the carb bowls are filled.  This should draw extra fuel from the mains needle jet to flood the cylinders.  Then kick with about 1/8 or less throttle.
You could also do a shop test with the coils turned off.  Close choke, kick four or five times with WOT and then pull plugs to see effect.  They should be wet with gas.   If not, you have to find out why the choke is ineffective.

[paulages]

Hard to picture where the legs are when riding.  Those are going to effect air flow, too, of course.
Not too hard to see where the inner carb inlets might get higher pressure than the outers, though.
Given any thought to adapting the stock air plenum? (I know, heresy...)

I suppose none of this has any relevance to start up issues, really.  If your choke plates don't give enough enrichment, there's really no point in making the idle circuits richer just for cold start.

Start procedure suggestion:
With the choke plates closed give'r wide open throttle for a kick or two after you know the carb bowls are filled.  This should draw extra fuel from the mains needle jet to flood the cylinders.  Then kick with about 1/8 or less throttle.
You could also do a shop test with the coils turned off.  Close choke, kick four or five times with WOT and then pull plugs to see effect.  They should be wet with gas.   If not, you have to find out why the choke is ineffective.

my knees are about two inches behind the tails of the fairing when riding. i don't believe the stock plenum will work with the 750 carbs, but maybe i could open the holes up to accept the 750 rubbers... that might work. i could then build a larger airbox to allow high flow. hmm... not a bad idea, lloyd.

my usual startup procedure us actually similar to what you describe. i learned it from having a triumph with no choke... on the honda i use full choke, kick the engine through a few times with WOT, and then reduce choke and kick with ignition on.

i'll do more tests as soon as i can get some clean D7EAs.

[paulages]

dragging out an old thread here...

finally got time to to some diagnostic on the bike. using a large battery (BMW i think) to ensure adequate voltage, i tested for spark again, only to find strange erratic spark. i decided to recheck the timing on the dyna 2000 pickup, only to find that one of the pickups had lost a screw and was hanging by the other, out of position. the pickup was close enough to allow occasional firing, but it was obvious this was my main problem. so much for "set and forget" with electronic ignition... 

re-timed that side and it fired up fairly easily on the 1.5 AH battery i run in my points-ignition BSA. on to fine tuning...

[turboguzzi]

now go stand in the corner!

[paulages]

now go stand in the corner!

seriously.. but wasn't that stupidity punishment enough?