i have all sort off issues with a friends 928 which is a team driver of ours and would really like to help out.
Basically i noticed his engine really rich and bogging due to the fuel buildup in the technical sections of the track. It even backfired sometimes and it did even shut of a lot.
TM928 Engine Spec
Orignal engine was the one for the HPI savage i.e. with rotostart back plate.
installed the Starter box back plate muffler 2045 Plug RB 7 & 6 Installed an 8mm Venturi to tame a bit the monster Cleaned the filter Fuel 20% Nitro 10% Oil
What i tried is the following:
We set the needles to factory setting, tried it out but it was way to rich so i started closing the HSN until i got a good temp and started tuning again the HSN until it felt right i.e. a good trial of smoke. Than i started tuning the LSN as it felt rich from the bottom. when i almost got to a close tune that is a tad rich on the bottom the engine started bogging and sometimes even shut off, sounded like it got flooded. Closed a bit more the LSN and it started back firing once again.
Went back to factory default and started all over again. When i started tuning the LSN it started to do the same thing. From my experience i thought it was a timing issue so i tried installing a new Turbo plug 7 fired it up but still exactly same issues. I also tried a Turbo plug 6 which resulted in the same issues but managed to hold an idle a little better.
We took it to my work shop dismantled the engine to pieces including the carb and inspected every part of it including orings. Everything looked good.
What i couldn't understand well is the head shims under the head button as on the manual it says 0.3 (0.6) which i assume that means that i have to find 2 0.3 shims, but i only found one.
We also replaced the conrod due that it was close to the gallon mark.
Fired it up again broke in the conrod and started tuning but still we had the same issue.
Could it be a head shim issue ?
Posts: 25 | From: Malta | Registered: Mar 2008
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posted
My best guess would be the tune... I would start over with the tune.... You will want to run pretty fat on the hsn... So as soon as the engine clears out and yyou get good performance, move onto the lsn, in which( from my exsperience), likes to be pretty lean...leave the hsn alone from here on...if the engine starts racing at idle, lower your idle gap. The idle gap can be very low on this engine, without it stalling...
It may also help to tune it without the carb restrictor at first... I have a horrible time tuning with them myself... I think they also narrow "the tuning window" that much more....
I'm not sure about the backfire you're talking about... The 928 does have a distinctive sound to it.... Almost like a ping or a pop... My guess, is just the nature of the beast... Probably from its timing..... Dunno..... This engine is a little harder to tune than most budget engines.... But you just have to take your time to learn it....
Posts: 67 | From: Covington, La. | Registered: May 2006
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posted
Thanks DawgBone for your help, yes actually i noticed that this engine doesn't like beeing run rich. tried every type of combo with the hsn i.e. i tried leaving the HSN a bit on the rich side than started closing the LSN but still with issues than i tried the other way round closed the HSN to its maximum performance and tuned the LSN afterwards but still with problems.
Re backfire it literally backfires, i mean i was tuning this engine with out airfilter on the starter box to check for idle etc... and when i went full throttle it was literraly spitting fuel from the carb ( spray )
You removed even the 9 venturi to tune it when you said to remove the carb restrictor ?
Thanks once again
Posts: 25 | From: Malta | Registered: Mar 2008
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posted
I did take out the 9mm insert...The carb is shaped like the venturi of the restrictors, with no blunt edges, so it won't effect the waythe engine runs so to speak...Taking the 9mm insert out will just help you see where you stand with the idle gap...It probably won't provide any more air either, as the carb is also 9mm...But taking them out, helped me get the ballpark tune I needed...Once I found the tune I needed, I later installed the insert, and tuned accordingly...
Spitting fuel out of the carb at WOT doesn't sound right...I haven't seen that since I first tried my hand at modifying a TRX motor, and grinding on the leading edge of the crank flute...It ran, but spat out more fuel than it sucked in...I believe that nitro engines do spray a little bit of fuel out of the carb, but from your description, it sounds like alot, and like something isn't right...Are you sure that you re-installed everything right....Or that the sleeve is completely seated...Is the pipe clear of any debri...like exhaust gasket chunks....I like to use an air compressor to blow through the stinger...Does the engine respond to your tuning?
I'm still leaning on the tuning...215F is way too cool IMO...From my exsperience, these engines do not tune in like other engines..I actually learned alot about engine tuning from my 1st 928...But 215F is too cold...and the fuel coming out of the carb could just be from being too rich...my 928 tunes in with a semi fat HSN....super lean LSN...very small idle gap...the pics may help for reference...
Try tuning the engine from a half tank to empty...Mine temps out at 260-265F at the very end of the tank...
Posts: 67 | From: Covington, La. | Registered: May 2006
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Will try it out tomorrow i also bought some shims so that i will try adding a .3 and see the engine reaction. How many shims do you have under your head ?
Posts: 25 | From: Malta | Registered: Mar 2008
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i would not recommend adding an additional .3mm shim to whats already installed with the head, if you do the engine will be over shimmed and the explosion will take place internally after the piston has reached top dead center resulting in less bottom ned power but more top speed.
you wont technically hurt the engine but if you are running 25-30% nitro tuning the engine might be a bit tricky
I would not add any shims that weren't there out of the box....It's developed out of the box for 25% nitro....which means it should run just fine, if not perfect, with either 20% or 30%...
215F is too cool, and I think there is more to tune on...Like I mentioned, tune on a half tank...and don't read so much into the temps...tune for performance....that's what's kewl about this engine, as it doesn't mind hotter temps(260F+)...it will let you know when it's reached it limits...so when it get's too hot...it will simply run like a dog/turd(lose power)....at that point, you should try to cool it down, re-adjust your tune(richen), then try again...
posted
You can find this information on page 2 of the exploded view supplied with your engine, if you cannot find this information then check out the support section of our website. You can find there the exploded view of your engine for download.
yes we did check the exploded view we had, but i think there is some sort of mistake because on the upper part ( engine exploded view ) it shows only one shim under the head with a product code which when we checked it was a 0.3mm one, than below it says that it comes with 2 x 0.3mm.
We tried the engine back again but still with lots of issues. Our last attempt is to install a 728 normal plug button head if its compatible i.e. please confirm.
Thanks & Regards
Ramon
Posts: 25 | From: Malta | Registered: Mar 2008
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posted
Yes, the TM728 button is compatible and you need 2x 0.3mm under the head-button.
On the exploded view you will find the part# of the shim and on page 2 the qty of these shims you need. Sorry if this has confused you but on some engines there is more then 2 head shims and if we would put them all then it will be more confusing so we therefor made on page 2 the summary for this.
The Normal head-button should make it easier to tune.
posted
Thanks Rody yes infact thats why i couldn't understand the shimming issue as on my other RB engine explded view manual it clearly shows more than on shim.
Thats great we will try the 728 button.
Posts: 25 | From: Malta | Registered: Mar 2008
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