Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Moderator: Moderators

Post Reply
Catalin24
Posts: 5
Joined: Sat Oct 07, 2017 7:36 pm
My Bike: ZXR 400 L5

Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Post by Catalin24 » Fri May 04, 2018 12:02 pm

Hello I need help with my ZXR 400 L5, it has quite a list of problems..

PLEASE FORGIVE ME FOR THE HUGE POST BUT I WANT TO EXPLAIN AS BETTER AS I CAN. PLEASE HELP! :rant


1. As stated in the title the engine runs "backfirey" throughout pretty much all the rpm range it cleans off a bit on 10k-14k. If I rev the engine with the throttle not too opened (i.e 20-25%) and steady the revs go up to the red line, but if try to open the throttle more (i.e 50-60%) the engine backfires like crazy over 9-10k rpm like its hitting the rev limiter. I don't know how to explain the noise really well and it's hard to record also as the exhaust it's pretty loud i would try to attach some clips. (it like... growly , it doesn't scream nicely). I though it was running rich and tried to re-adjust the mixture(tried pretty much everything, 1,75/2,0/2,5/3,0 turns) didn't make such a difference so i swaped the carbs with "new" ones. Mine had quite a play in the sliders and i thought they had wear on the needle jets, and needle jets seats. I striped down and cleaned the new one well(i think) didn't messed with the mixture-screws as i was afraid I won't be able to get the mixture right again like on the old carbs. Looked at the sparkplugs they seem to be decent, only number 2 a bit to rich and number 3 Maybe too lean? I adjusted the no.2 and no.3 mixtures just a tad accordingly. I was told that the carbs were balanced (so i don't think they are really really bad out of sync, i know my engine most likely has different compression on cylinders)

2. It wont idle below 2k rpm, it will stay at 1500 for a moment they almost stall at 1000. Adjusting the idle screw below 2k won't affect the idle until it the throttle bodies are almost closed and I think that's why it would almost stall.

3. Once it gets hot (middle line, first line above middle) the engine it's hard too start, some times it won't start at all. I need to give the throttle a good yank to get it started. Runs the same after it starts.

4.Lot's of pops and crackles, and some backfires when riding and decelerating.

5. Revs seem to raise kinda slow, it's not shooting on the rev range like it did when I mounted the cleaned carbs. It ran ran a lot better the day I mounted them. (maybe a week ago, the bike didn't see to much action)

6. Sometime the battery just dies, no obvious reason why. Even with the engine running, you will stop the bike and the battery would be almost empty. I checked the alternator coils(all 3 seem to be ok, in the range of 2-8ohm) all 3 are almost the same on voltage generated at 4k rpm ~49-51V
Check the voltage at the battery terminals at idle its above 14v but when reving the enigne it drops till ~13,8V. In the manual it states that it should be 14-15V all the time. The not charging issue is kinda intermitent took the next bike for the same shot trip, the battery was fine. Started with 13.01 got back with 13.40V

What i did to the engine:
-new plugs NGK CR9EK (double electrods)
-Looked at the air filter, seemd to be in a decent condition
-Played with the air/fuel mixture by opening the air intake and closing it. Took of the snorkle that is on air filter access hole. It did not help with the backfires even if I blocked the intake almost completly (just stalled the engine)
-The battery is new, AGM VARTA 8Ah Holds charge ok
-Verified all the fuses, all ok
-Tried to verify the rec/reg with the multimeter with the ohm method stated in the manual. Couldn't get a definitive reading.
-The engine was "rebuild" on a budget by me with the manual. Changed the head gasket as it was blown on no.2, 3 and 4. Head and block skimmed. base gasket replaced as well. But it seems to leak oil from the gap between the head and the block, it's not a huge leak though only gets that part of the engine a bit damp.
-I replaced the timing chain as it was worn, could've i got the timing wrong? The valves are not hitting the pistons I'm quite sure of that, but maybe they are not closing properly? I timed the engine as described in the manual. But recently I've found a post that made me think maybe i've got the cams off by 180* , would the engine run like that without blowing it to pieces? Also the timing plate with the mark and the little stud for the crank position sensor has play in it as somebody bent a bit the location pin on the crankshaft and its a bit smaller now. When mounted it I did my best to hold the plate positioned correctly on the crankshaft and dismiss the possibility of it being off by a few degrees. Also my thinking is that maybe I didn't put back the timing plate in the right spot as it has play, even though I tried to get it right. (hold it on the part of the pin that wasn't bent, as the pin is now smaller diameter and the plate has play) And maybe the spark is too soon or too late?

Maybe it needs valve adjustment? I did not made a compression test but I really want to do it now. (cold dry, cold wet and when hot) and check the valve clearances.

List to do:
-Compresion test
-Check valves and adjust
-Check coils and HT leads

https://ibb.co/hXKXDn
https://ibb.co/inzAzS
https://ibb.co/m333KS
Last edited by Catalin24 on Fri May 04, 2018 12:28 pm, edited 1 time in total.

Catalin24
Posts: 5
Joined: Sat Oct 07, 2017 7:36 pm
My Bike: ZXR 400 L5

Re: Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Post by Catalin24 » Fri May 04, 2018 12:07 pm

I"ve forgot to say that without the airbox the engine runs really lean(revs hang). With the airbox runs as described. Maybe it's too lean? although the spark plugs are quite black.

LE: Found how to edit post. :pmsl

Video of idling and revs below 8k : https://youtu.be/58Jm8D8aI2s
Video of revs above 8k(first time after carb rebuild, mostly unused plugs. Now it runs a lot worse. It still backfired now and then, but now it's like a machine gun almost everytime): https://www.youtube.com/watch?v=ecfVaScRr1U

Catalin24
Posts: 5
Joined: Sat Oct 07, 2017 7:36 pm
My Bike: ZXR 400 L5

Re: Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Post by Catalin24 » Sat May 05, 2018 8:42 pm

I've got news! Done some checking today and found this, no. 3 HT lead had improper contact on the coil and both secondary windings from the coils are out of spec.
No 1 and 4 coil : Primary 3,2Ohm(in spec) Secondary 24,3kOhm(out of spec)
No 2 and 3 coil : Primary 3,2Ohm(in spec) Secondary 24,1kOhm(out of spec)

My question would be, are my HT leads bad or are my coils bad? I didn't wanted to remove them from the bike, it's a bit to much hassle to get them out because are are zip tied on. The rubber mounts were bad, couldn't find a better and easy solution. Works for now I guess. I did check the no. 3 HT lead it had 4,6kOhm of ressitance. Is it normal?

Carb mixture screw adjustment were:
No1: 4,75 Out
No2: 4,60 Out
No3: 4,50 Out
No4: 4,75 Out

As banner told be the bike was running too rich. I've put them on ~1,75 turns out and the revs hanged a bit and I gave her a bit more gas. In the end it's at ~2,1 turns out. Still backfires around 10k If the throttle is yanked hard, still doesn't really want to idle nicely below 2k. BUT it runs a lot better, starts when hot and goes to 1500rpm but it doesn't really seem to be happy idling there If I blip a bit the throttle it goes back to 2k.

As for the "engine pull factor" it does seem to pull a lot harder. Still pop's and crackles like crazy when decelerating.
In the end, I'm quite happy with the result. The bike is rideable.

User avatar
banner001
Track day God
Track day God
Posts: 2187
Joined: Sat Sep 24, 2011 3:27 pm
My Bike: ZXR400 L4
Location: Nuneaton

Re: Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Post by banner001 » Sat May 05, 2018 10:16 pm

Sounds good, though backing off your mixture screw from 1.75 to 2 turns will add in more fuel/air mixture for a given engine rpm (i.e. a righer mixture)...

i did a review on the zxr250 site of how to dismantle and repair your coils...the process is identical for the zxr400, other than the 250 HT lead being inline with the cap, and on the 400 its at a ~70-degree angle.

as for your results, your primarys are good, so no issues with the coils, personally i would say its a bit of corrosion in the plug cap...i would also recommend changing the HT leads at the same time, they are cheap, and cleaning the plug caps is free...you can test these individually as you go along, i.e. remove a plug cap and measure its resistance from the screw where the HT lead goes to where the spark plug contacts, this number should be 15kOhm, if its any higher than this then your plug needs cleaning, if its 15kOhm then it was a bad HT lead.
UK ZXR 400 L4
UK ZXR 400 L3 track bike
1989 FZR 1000 EXUP-R streetfighter

Catalin24
Posts: 5
Joined: Sat Oct 07, 2017 7:36 pm
My Bike: ZXR 400 L5

Re: Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Post by Catalin24 » Sun May 06, 2018 11:11 am

I turned them out to ~2 turns out because it seemed to me that it was a bit too lean on the pilot circuit. i.e the bike hanged around 3k rpm and then drop to 2k. Maybe I am wrong, please correct me.

Could you provide me a link for that coil rebuild? I can't find it.

I did attempted to clean the spark plug caps with some WD-40, ran out of contact cleaner. But the spark plug caps we're cleaned with contact cleaner last year when I did a harness clean. Maybe I did not get them clean enough, they didn't looked green or white inside. Only the usual old brass look, a bit faded. (not shiny)

As for what to measure you mean only the spark plug cap, the part that goes on the plug right? And it has to be 15kOhm without the lead going to the coil. Did I understood you correctly?

I'm gonna buy some HT lead as is really cheap only 0.90 pounds for 1 meter. I need to make them the exact same length right?

The compression test is still on hold, but it will be done.

User avatar
banner001
Track day God
Track day God
Posts: 2187
Joined: Sat Sep 24, 2011 3:27 pm
My Bike: ZXR400 L4
Location: Nuneaton

Re: Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Post by banner001 » Sun May 06, 2018 12:19 pm

Sorry, thought I had put the link in...

https://2fiftycc.com/index.php?threads/ ... ost-109603

You might need to register to get the pictures in a larger size, basically follow that and you will be good.

I would make the HT lead about 1" longer than it previously was, this will allow you in the future to trim off a small amount from where it screws into the plug caps if necessary, without cutting a fresh length of HTLead
UK ZXR 400 L4
UK ZXR 400 L3 track bike
1989 FZR 1000 EXUP-R streetfighter

Catalin24
Posts: 5
Joined: Sat Oct 07, 2017 7:36 pm
My Bike: ZXR 400 L5

Re: Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Post by Catalin24 » Sun May 06, 2018 8:29 pm

I've read the guide and now I'm in a bit of confusion. The manual for zxr 400 states that the secondary windings resistance should be between 12 - 18 kOhm and mines are both around 24 kOhm. But in your guide the difference was huge on one of the coils from 87kOhm to 23kOhm. I know they are for the 250 but still..

Could my coils and HT leads be good and the backfires on high throttle only when reving at a standstill come from other cause? Maybe it's lean on the WOT circuit? (needle jet adjustment?)

Also I've seen a bit of rambling about "Blue M" unrestricted CDI's . Mine it's a Denso and it has a big "M" on it with blue writing. I'm the lucky owner of an unrestricted CDI ? What does it really make? :smt017

User avatar
banner001
Track day God
Track day God
Posts: 2187
Joined: Sat Sep 24, 2011 3:27 pm
My Bike: ZXR400 L4
Location: Nuneaton

Re: Engine runs "backfirey". Backfires at high rpm. Hard to start when hot

Post by banner001 » Tue May 08, 2018 12:13 am

in the manual, the 12-18kohm is wit hte plug cap removed from the HT lead, i.e. plug cap to plug cap should be abot 10kOhm higher, so 22-28kOhm, so if yours are 24kOhm plug cap to plug cap then they are fine.

mine were from a 30 year old bike that hadnt been started in 13 years, and left in a garage...

as for the CDi, stick with the blue M spec, its unrestricted and should be the best for your bike.
UK ZXR 400 L4
UK ZXR 400 L3 track bike
1989 FZR 1000 EXUP-R streetfighter

Post Reply